diff --git a/src/object/motion/motionant.cpp b/src/object/motion/motionant.cpp
index f96da8dc..ab1e2398 100644
--- a/src/object/motion/motionant.cpp
+++ b/src/object/motion/motionant.cpp
@@ -27,6 +27,8 @@
 
 #include "object/old_object.h"
 
+#include "object/subclass/base_alien.h"
+
 #include "physics/physics.h"
 
 
@@ -427,7 +429,7 @@ bool CMotionAnt::EventFrame(const Event &event)
     assert(m_object->Implements(ObjectInterfaceType::Destroyable));
     if ( dynamic_cast<CDestroyableObject*>(m_object)->GetDying() == DeathType::Burning )  // burning?
     {
-        if ( m_object->GetFixed() )
+        if ( dynamic_cast<CBaseAlien*>(m_object)->GetFixed() )
         {
             m_actionType = MAS_BURN;
         }
@@ -722,7 +724,7 @@ bool CMotionAnt::EventFrame(const Event &event)
         if ( m_progress >= 1.0f )
         {
             SetAction(-1);
-            m_object->SetFixed(false);  // moving again
+            dynamic_cast<CBaseAlien*>(m_object)->SetFixed(false);  // moving again
         }
     }
     else
diff --git a/src/object/motion/motionspider.cpp b/src/object/motion/motionspider.cpp
index 8bad1264..4c7c1b4c 100644
--- a/src/object/motion/motionspider.cpp
+++ b/src/object/motion/motionspider.cpp
@@ -27,6 +27,8 @@
 
 #include "object/old_object.h"
 
+#include "object/subclass/base_alien.h"
+
 #include "physics/physics.h"
 
 #include <stdio.h>
@@ -362,7 +364,7 @@ bool CMotionSpider::EventFrame(const Event &event)
     assert(m_object->Implements(ObjectInterfaceType::Destroyable));
     if (dynamic_cast<CDestroyableObject*>(m_object)->GetDying() == DeathType::Burning )  // burning?
     {
-        if ( m_object->GetFixed() )
+        if ( dynamic_cast<CBaseAlien*>(m_object)->GetFixed() )
         {
             m_actionType = MSS_BURN;
         }
@@ -646,7 +648,7 @@ bool CMotionSpider::EventFrame(const Event &event)
         if ( m_progress >= 1.0f )
         {
             SetAction(-1);
-            m_object->SetFixed(false);  // moving again
+            dynamic_cast<CBaseAlien*>(m_object)->SetFixed(false);  // moving again
         }
     }
     else
diff --git a/src/object/old_object.cpp b/src/object/old_object.cpp
index 9f92ce3b..6081c329 100644
--- a/src/object/old_object.cpp
+++ b/src/object/old_object.cpp
@@ -51,6 +51,7 @@
 #include "object/motion/motion.h"
 #include "object/motion/motionvehicle.h"
 
+#include "object/subclass/base_alien.h"
 #include "object/subclass/exchange_post.h"
 
 #include "physics/physics.h"
@@ -134,7 +135,6 @@ COldObject::COldObject(int id)
     m_bTrainer = false;
     m_bToy = false;
     m_bManual = false;
-    m_bFixed = false;
     m_aTime = 0.0f;
     m_shotTime = 0.0f;
     m_bVirusMode = false;
@@ -961,11 +961,6 @@ void COldObject::Write(CLevelParserLine* line)
     if ( !GetSelectable() )
         line->AddParam("selectable", MakeUnique<CLevelParserParam>(GetSelectable()));
 
-    // TODO: doesn't seem to be used
-    // But it is, this is used by aliens after Thumper ~krzys_h
-    if ( GetFixed() )
-        line->AddParam("fixed", MakeUnique<CLevelParserParam>(GetFixed()));
-
     if ( !GetCollisions() )
         line->AddParam("clip", MakeUnique<CLevelParserParam>(GetCollisions()));
 
@@ -1139,7 +1134,6 @@ void COldObject::Read(CLevelParserLine* line)
     // Everthing below is for use only by saved scenes
     if (line->GetParam("energy")->IsDefined())
         SetEnergyLevel(line->GetParam("energy")->AsFloat());
-    SetFixed(line->GetParam("fixed")->AsBool(false));
     SetLock(line->GetParam("lock")->AsBool(false));
     SetGunGoalV(line->GetParam("aimV")->AsFloat(0.0f));
     SetGunGoalH(line->GetParam("aimH")->AsFloat(0.0f));
@@ -2048,20 +2042,25 @@ bool COldObject::EventProcess(const Event &event)
             {
                 axeZ = -1.0f;  // tomb
 
-                if ( !GetFixed() &&
-                     (m_type == OBJECT_ANT    ||
+                if ( (m_type == OBJECT_ANT    ||
                       m_type == OBJECT_SPIDER ||
                       m_type == OBJECT_WORM   ) )
                 {
-                    axeY = 2.0f;  // zigzag disorganized fast
-                    if ( m_type == OBJECT_WORM )  axeY = 5.0f;
-                    axeX = 0.5f+sinf(m_time* 1.0f)*0.5f+
-                                sinf(m_time* 6.0f)*2.0f+
-                                sinf(m_time*21.0f)*0.2f;
-                    float factor = 1.0f-m_burnTime/15.0f;  // slow motion
-                    if ( factor < 0.0f )  factor = 0.0f;
-                    axeY *= factor;
-                    axeX *= factor;
+                    // TODO: Move to CBaseAlien?
+                    CBaseAlien* alien = dynamic_cast<CBaseAlien*>(this);
+                    assert(alien != nullptr);
+                    if (!alien->GetFixed())
+                    {
+                        axeY = 2.0f;  // zigzag disorganized fast
+                        if ( m_type == OBJECT_WORM )  axeY = 5.0f;
+                        axeX = 0.5f+sinf(m_time* 1.0f)*0.5f+
+                                    sinf(m_time* 6.0f)*2.0f+
+                                    sinf(m_time*21.0f)*0.2f;
+                        float factor = 1.0f-m_burnTime/15.0f;  // slow motion
+                        if ( factor < 0.0f )  factor = 0.0f;
+                        axeY *= factor;
+                        axeX *= factor;
+                    }
                 }
             }
             m_physics->SetMotorSpeedX(axeY);  // move forward/move back
@@ -2537,18 +2536,6 @@ void COldObject::SetTransparency(float value)
     }
 }
 
-// Indicates whether an object is stationary (ant on the back).
-
-void COldObject::SetFixed(bool bFixed)
-{
-    m_bFixed = bFixed;
-}
-
-bool COldObject::GetFixed()
-{
-    return m_bFixed;
-}
-
 
 // Pushes an object.
 
diff --git a/src/object/old_object.h b/src/object/old_object.h
index 9787c694..0706fe92 100644
--- a/src/object/old_object.h
+++ b/src/object/old_object.h
@@ -210,9 +210,6 @@ public:
 
     void        SetTransparency(float value) override;
 
-    void        SetFixed(bool bFixed) override;
-    bool        GetFixed() override;
-
     Math::Sphere GetJostlingSphere() const override;
     bool        JostleObject(float force) override;
 
@@ -360,7 +357,6 @@ protected:
     bool        m_bTrainer;         // drive vehicle (without remote)
     bool        m_bToy;             // toy key
     bool        m_bManual;          // manual control (Scribbler)
-    bool        m_bFixed;
     float       m_gunGoalV;
     float       m_gunGoalH;
     Gfx::CameraType  m_cameraType;
diff --git a/src/object/old_object_interface.cpp b/src/object/old_object_interface.cpp
index e65818f7..4d5bb13d 100644
--- a/src/object/old_object_interface.cpp
+++ b/src/object/old_object_interface.cpp
@@ -95,17 +95,6 @@ Character* COldObjectInterface::GetCharacter()
 }
 
 
-void COldObjectInterface::SetFixed(bool bFixed)
-{
-    throw std::logic_error("SetFixed: not implemented!");
-}
-
-bool COldObjectInterface::GetFixed()
-{
-    throw std::logic_error("GetFixed: not implemented!");
-}
-
-
 void COldObjectInterface::SetVirusMode(bool bEnable)
 {
     throw std::logic_error("SetVirusMode: not implemented!");
diff --git a/src/object/old_object_interface.h b/src/object/old_object_interface.h
index a372e8ea..5a3e72ca 100644
--- a/src/object/old_object_interface.h
+++ b/src/object/old_object_interface.h
@@ -81,10 +81,6 @@ public:
 
     virtual void        FlatParent();
 
-    // This goes to CBaseAlien or something like that
-    virtual void        SetFixed(bool bFixed);
-    virtual bool        GetFixed();
-
     // Not sure. Maybe a separate interface, or maybe CControllableObject (buildings can have viruses too)
     virtual void        SetVirusMode(bool bEnable);
     virtual bool        GetVirusMode();
diff --git a/src/object/subclass/base_alien.cpp b/src/object/subclass/base_alien.cpp
index 7efc404c..d1f2f156 100644
--- a/src/object/subclass/base_alien.cpp
+++ b/src/object/subclass/base_alien.cpp
@@ -21,6 +21,9 @@
 
 #include "common/make_unique.h"
 
+#include "level/parser/parserline.h"
+#include "level/parser/parserparam.h"
+
 #include "graphics/engine/oldmodelmanager.h"
 
 #include "object/object_create_params.h"
@@ -35,7 +38,8 @@
 
 
 CBaseAlien::CBaseAlien(int id, ObjectType type)
-    : CBaseVehicle(id, type)
+    : CBaseVehicle(id, type),
+      m_fixed(false)
 {}
 
 CBaseAlien::~CBaseAlien()
@@ -85,3 +89,28 @@ std::unique_ptr<CBaseAlien> CBaseAlien::Create(
 
     return std::move(obj);
 }
+
+void CBaseAlien::SetFixed(bool fixed)
+{
+    m_fixed = fixed;
+}
+
+bool CBaseAlien::GetFixed()
+{
+    return m_fixed;
+}
+
+void CBaseAlien::Read(CLevelParserLine* line)
+{
+    COldObject::Read(line);
+
+    SetFixed(line->GetParam("fixed")->AsBool(false));
+}
+
+void CBaseAlien::Write(CLevelParserLine* line)
+{
+    COldObject::Write(line);
+
+    if (GetFixed())
+        line->AddParam("fixed", MakeUnique<CLevelParserParam>(GetFixed()));
+}
diff --git a/src/object/subclass/base_alien.h b/src/object/subclass/base_alien.h
index 39f9bc85..4e0c8e27 100644
--- a/src/object/subclass/base_alien.h
+++ b/src/object/subclass/base_alien.h
@@ -44,4 +44,17 @@ public:
         const ObjectCreateParams& params,
         Gfx::COldModelManager* modelManager,
         Gfx::CEngine* engine);
+
+public:
+    //! Management of "temporarirly stationary" mode (alien on the back)
+    //@{
+    void SetFixed(bool fixed);
+    bool GetFixed();
+    //@}
+
+    void Write(CLevelParserLine* line) override;
+    void Read(CLevelParserLine* line) override;
+
+protected:
+    bool m_fixed;
 };
diff --git a/src/object/task/taskadvance.cpp b/src/object/task/taskadvance.cpp
index 201fffe5..a066d8f3 100644
--- a/src/object/task/taskadvance.cpp
+++ b/src/object/task/taskadvance.cpp
@@ -25,9 +25,10 @@
 #include "math/geometry.h"
 
 #include "object/object.h"
-
 #include "object/old_object.h"
 
+#include "object/subclass/base_alien.h"
+
 #include "physics/physics.h"
 
 
@@ -56,7 +57,8 @@ bool CTaskAdvance::EventProcess(const Event &event)
     m_fixTime += event.rTime;
 
     // Momentarily stationary object (ant on the back)?
-    if ( m_object->GetFixed() )
+    CBaseAlien* alien = dynamic_cast<CBaseAlien*>(m_object);
+    if ( alien != nullptr && alien->GetFixed() )
     {
         m_physics->SetMotorSpeedX(0.0f);  // stops the advance
         m_physics->SetMotorSpeedZ(0.0f);  // stops the rotation
diff --git a/src/object/task/taskfireant.cpp b/src/object/task/taskfireant.cpp
index 483649b9..b95b22d3 100644
--- a/src/object/task/taskfireant.cpp
+++ b/src/object/task/taskfireant.cpp
@@ -28,6 +28,8 @@
 
 #include "object/motion/motionant.h"
 
+#include "object/subclass/base_alien.h"
+
 #include "physics/physics.h"
 
 
@@ -58,7 +60,7 @@ bool CTaskFireAnt::EventProcess(const Event &event)
     if ( event.type != EVENT_FRAME )  return true;
     if ( m_bError )  return false;
 
-    if ( m_object->GetFixed() )  // insect on its back?
+    if ( dynamic_cast<CBaseAlien*>(m_object)->GetFixed() )  // insect on its back?
     {
         m_bError = true;
         return false;
@@ -98,7 +100,7 @@ Error CTaskFireAnt::Start(Math::Vector impact)
     if ( type != OBJECT_ANT )  return ERR_WRONG_BOT;
 
     // Insect on its back?
-    if ( m_object->GetFixed() )  return ERR_WRONG_BOT;
+    if ( dynamic_cast<CBaseAlien*>(m_object)->GetFixed() )  return ERR_WRONG_BOT;
 
     m_physics->SetMotorSpeed(Math::Vector(0.0f, 0.0f, 0.0f));
 
@@ -128,7 +130,7 @@ Error CTaskFireAnt::IsEnded()
 
     if ( m_engine->GetPause() )  return ERR_CONTINUE;
     if ( m_bError )  return ERR_STOP;
-    if ( m_object->GetFixed() )  return ERR_STOP;  // insect on its back?
+    if ( dynamic_cast<CBaseAlien*>(m_object)->GetFixed() )  return ERR_STOP;  // insect on its back?
 
     if ( m_phase == TFA_TURN )  // rotation ?
     {
diff --git a/src/object/task/taskgoto.cpp b/src/object/task/taskgoto.cpp
index c47a97d2..a4df61fd 100644
--- a/src/object/task/taskgoto.cpp
+++ b/src/object/task/taskgoto.cpp
@@ -34,6 +34,8 @@
 
 #include "object/interface/transportable_object.h"
 
+#include "object/subclass/base_alien.h"
+
 #include "physics/physics.h"
 
 #include <string.h>
@@ -79,7 +81,8 @@ bool CTaskGoto::EventProcess(const Event &event)
     if ( event.type != EVENT_FRAME )  return true;
 
     // Momentarily stationary object (ant on the back)?
-    if ( m_object->GetFixed() )
+    CBaseAlien* alien = dynamic_cast<CBaseAlien*>(m_object);
+    if ( alien != nullptr && alien->GetFixed() )
     {
         m_physics->SetMotorSpeedX(0.0f);  // stops the advance
         m_physics->SetMotorSpeedZ(0.0f);  // stops the rotation
diff --git a/src/object/task/taskspiderexplo.cpp b/src/object/task/taskspiderexplo.cpp
index 87966b0a..28eb3454 100644
--- a/src/object/task/taskspiderexplo.cpp
+++ b/src/object/task/taskspiderexplo.cpp
@@ -27,6 +27,8 @@
 
 #include "object/motion/motionspider.h"
 
+#include "object/subclass/base_alien.h"
+
 #include "physics/physics.h"
 
 
@@ -55,7 +57,7 @@ bool CTaskSpiderExplo::EventProcess(const Event &event)
     if ( event.type != EVENT_FRAME )  return true;
 
     // Momentarily stationary object (ant on the back)?
-    if ( m_object->GetFixed() )
+    if ( dynamic_cast<CBaseAlien*>(m_object)->GetFixed() )
     {
         m_bError = true;
         return true;
diff --git a/src/object/task/taskterraform.cpp b/src/object/task/taskterraform.cpp
index e4c53979..753c2f0d 100644
--- a/src/object/task/taskterraform.cpp
+++ b/src/object/task/taskterraform.cpp
@@ -35,6 +35,8 @@
 #include "object/motion/motionant.h"
 #include "object/motion/motionspider.h"
 
+#include "object/subclass/base_alien.h"
+
 #include "physics/physics.h"
 
 
@@ -377,21 +379,16 @@ bool CTaskTerraform::Terraform()
             dist = Math::Distance(m_terraPos, pObj->GetPosition());
             if ( dist > ACTION_RADIUS )  continue;
 
-            if ( type == OBJECT_ANT )
+            if ( type == OBJECT_ANT || type == OBJECT_SPIDER )
             {
                 assert(pObj->Implements(ObjectInterfaceType::TaskExecutor));
                 dynamic_cast<CTaskExecutorObject*>(pObj)->StopForegroundTask();
 
-                motion->SetAction(MAS_BACK1, 0.8f+Math::Rand()*0.3f);
-                pObj->SetFixed(true);  // not moving
-            }
-            if ( type == OBJECT_SPIDER )
-            {
-                assert(pObj->Implements(ObjectInterfaceType::TaskExecutor));
-                dynamic_cast<CTaskExecutorObject*>(pObj)->StopForegroundTask();
-
-                motion->SetAction(MSS_BACK1, 0.8f+Math::Rand()*0.3f);
-                pObj->SetFixed(true);  // not moving
+                int actionType = -1;
+                if (type == OBJECT_ANT)    actionType = MAS_BACK1;
+                if (type == OBJECT_SPIDER) actionType = MSS_BACK1;
+                motion->SetAction(actionType, 0.8f+Math::Rand()*0.3f);
+                dynamic_cast<CBaseAlien*>(pObj)->SetFixed(true);  // not moving
             }
         }
     }
diff --git a/src/object/task/taskturn.cpp b/src/object/task/taskturn.cpp
index 53507015..89284a45 100644
--- a/src/object/task/taskturn.cpp
+++ b/src/object/task/taskturn.cpp
@@ -26,6 +26,8 @@
 
 #include "object/old_object.h"
 
+#include "object/subclass/base_alien.h"
+
 #include "physics/physics.h"
 
 
@@ -50,7 +52,8 @@ bool CTaskTurn::EventProcess(const Event &event)
     if ( event.type != EVENT_FRAME )  return true;
 
     // Momentarily stationary object (ant on the back)?
-    if ( m_object->GetFixed() )
+    CBaseAlien* alien = dynamic_cast<CBaseAlien*>(m_object);
+    if ( alien != nullptr && alien->GetFixed() )
     {
         m_physics->SetMotorSpeedX(0.0f);  // stops the advance
         m_physics->SetMotorSpeedZ(0.0f);  // stops the rotation
diff --git a/src/physics/physics.cpp b/src/physics/physics.cpp
index 06e93a03..4a678a91 100644
--- a/src/physics/physics.cpp
+++ b/src/physics/physics.cpp
@@ -52,6 +52,8 @@
 #include "object/motion/motionhuman.h"
 #include "object/motion/motionvehicle.h"
 
+#include "object/subclass/base_alien.h"
+
 #include "object/task/task.h"
 
 
@@ -1605,7 +1607,7 @@ void CPhysics::SoundMotor(float rTime)
     {
         assert(m_object->Implements(ObjectInterfaceType::Destroyable));
         if ( dynamic_cast<CDestroyableObject*>(m_object)->GetDying() == DeathType::Burning ||
-             m_object->GetFixed() )
+             dynamic_cast<CBaseAlien*>(m_object)->GetFixed() )
         {
             if ( m_lastSoundInsect <= 0.0f )
             {
@@ -1669,7 +1671,7 @@ void CPhysics::SoundMotor(float rTime)
     {
         assert(m_object->Implements(ObjectInterfaceType::Destroyable));
         if ( dynamic_cast<CDestroyableObject*>(m_object)->GetDying() == DeathType::Burning ||
-             m_object->GetFixed() )
+             dynamic_cast<CBaseAlien*>(m_object)->GetFixed() )
         {
             if ( m_lastSoundInsect <= 0.0f )
             {
diff --git a/src/script/scriptfunc.cpp b/src/script/scriptfunc.cpp
index 3da5f645..e04c63ba 100644
--- a/src/script/scriptfunc.cpp
+++ b/src/script/scriptfunc.cpp
@@ -50,6 +50,7 @@
 #include "object/interface/task_executor_object.h"
 #include "object/interface/trace_drawing_object.h"
 
+#include "object/subclass/base_alien.h"
 #include "object/subclass/exchange_post.h"
 
 #include "object/task/taskinfo.h"
@@ -2695,7 +2696,7 @@ bool CScriptFunctions::rMotor(CBotVar* var, CBotVar* result, int& exception, voi
     if ( turn < -1.0f )  turn = -1.0f;
     if ( turn >  1.0f )  turn =  1.0f;
 
-    if ( pThis->GetFixed() )  // ant on the back?
+    if ( dynamic_cast<CBaseAlien*>(pThis) != nullptr && dynamic_cast<CBaseAlien*>(pThis)->GetFixed() )  // ant on the back?
     {
         speed = 0.0f;
         turn  = 0.0f;