diff --git a/OpenSim/Region/Physics/BulletSPlugin/BSPhysObject.cs b/OpenSim/Region/Physics/BulletSPlugin/BSPhysObject.cs
index f89b376543..7a4655071d 100755
--- a/OpenSim/Region/Physics/BulletSPlugin/BSPhysObject.cs
+++ b/OpenSim/Region/Physics/BulletSPlugin/BSPhysObject.cs
@@ -462,7 +462,10 @@ public abstract class BSPhysObject : PhysicsActor
 
         // If someone has subscribed for collision events log the collision so it will be reported up
         if (SubscribedEvents()) {
-            CollisionCollection.AddCollider(collidingWith, new ContactPoint(contactPoint, contactNormal, pentrationDepth));
+            lock (PhysScene.CollisionLock)
+            {
+                CollisionCollection.AddCollider(collidingWith, new ContactPoint(contactPoint, contactNormal, pentrationDepth));
+            }
             DetailLog("{0},{1}.Collison.AddCollider,call,with={2},point={3},normal={4},depth={5},colliderMoving={6}",
                             LocalID, TypeName, collidingWith, contactPoint, contactNormal, pentrationDepth, ColliderIsMoving);
 
@@ -474,12 +477,14 @@ public abstract class BSPhysObject : PhysicsActor
     // Send the collected collisions into the simulator.
     // Called at taint time from within the Step() function thus no locking problems
     //      with CollisionCollection and ObjectsWithNoMoreCollisions.
+    // Called with BSScene.CollisionLock locked to protect the collision lists.
     // Return 'true' if there were some actual collisions passed up
     public virtual bool SendCollisions()
     {
         bool ret = true;
 
-        // If the 'no collision' call, force it to happen right now so quick collision_end
+        // If no collisions this call but there were collisions last call, force the collision
+        //     event to be happen right now so quick collision_end.
         bool force = (CollisionCollection.Count == 0 && CollisionsLastReported.Count != 0);
 
         // throttle the collisions to the number of milliseconds specified in the subscription
diff --git a/OpenSim/Region/Physics/BulletSPlugin/BSScene.cs b/OpenSim/Region/Physics/BulletSPlugin/BSScene.cs
index 23bada9927..17d26a98c2 100644
--- a/OpenSim/Region/Physics/BulletSPlugin/BSScene.cs
+++ b/OpenSim/Region/Physics/BulletSPlugin/BSScene.cs
@@ -705,7 +705,10 @@ public sealed class BSScene : PhysicsScene, IPhysicsParameters
     //    this is is under UpdateLock.
     public void PostUpdate(BSPhysObject updatee)
     {
-        ObjectsWithUpdates.Add(updatee);
+        lock (UpdateLock)
+        {
+            ObjectsWithUpdates.Add(updatee);
+        }
     }
 
     // The simulator thinks it is physics time so return all the collisions and position
@@ -803,7 +806,10 @@ public sealed class BSScene : PhysicsScene, IPhysicsParameters
             if (collider.Collide(collidingWith, collidee, collidePoint, collideNormal, penetration))
             {
                 // If a collision was 'good', remember to send it to the simulator
-                ObjectsWithCollisions.Add(collider);
+                lock (CollisionLock)
+                {
+                    ObjectsWithCollisions.Add(collider);
+                }
             }
         }