diff --git a/io_import_scene_mhx.py b/io_import_scene_mhx.py
index a395de215e991b2cf57c15d6ef64e4d64be54630..4004977caa4b0faa0f01888142076b2d03893f5b 100644
--- a/io_import_scene_mhx.py
+++ b/io_import_scene_mhx.py
@@ -3000,6 +3000,7 @@ class ImportMhx(bpy.types.Operator, ImportHelper):
     bl_label = "Import MHX"
     bl_space_type = "PROPERTIES"
     bl_region_type = "WINDOW"
+    bl_options = {'UNDO'}
 
     scale = FloatProperty(name="Scale", description="Default meter, decimeter = 1.0", default = theScale)
     enums = []
diff --git a/io_mesh_stl/__init__.py b/io_mesh_stl/__init__.py
index a46693210beea105fb0ebf8fafbfd757e0ee22a3..9a40de11ccb9f58f3bb2fe312236d6322c57c274 100644
--- a/io_mesh_stl/__init__.py
+++ b/io_mesh_stl/__init__.py
@@ -67,6 +67,7 @@ class ImportSTL(bpy.types.Operator, ImportHelper):
     '''Load STL triangle mesh data'''
     bl_idname = "import_mesh.stl"
     bl_label = "Import STL"
+    bl_options = {'UNDO'}
 
     filename_ext = ".stl"
 
diff --git a/io_scene_m3/import_m3.py b/io_scene_m3/import_m3.py
index 7a7678e89334a38885fa35402e5c91dd5bedf9a8..a6f48cd46d362f0c62a8977296f4ee5bf1d22232 100644
--- a/io_scene_m3/import_m3.py
+++ b/io_scene_m3/import_m3.py
@@ -341,6 +341,7 @@ class M3Importer(bpy.types.Operator):
     '''Import from M3 file format (.m3)'''
     bl_idname = "import_mesh.blizzard_m3"
     bl_label = 'Import M3'
+    bl_options = {'UNDO'}
 
     # List of operator properties, the attributes will be assigned
     # to the class instance from the operator settings before calling.