If you face a error of `ModuleNotFoundError: No module named 'robot_bases'` on pybullet 2.5.0,
you should **modify** a source code of pybullet.
This issue may be fixed on future release because the pybullet source code is updated on github.

If you are using anaconda, you need to modify `/anaconda3/envs/XXXX/lib/python3.6/site-packages/pybullet_envs/robot_locomotors.py`.

Please open robot_locomotors.py.
You can see...

```robot_locomotors.py
 from robot_bases import XmlBasedRobot, MJCFBasedRobot, URDFBasedRobot
 import numpy as np
 import pybullet
 import os
 import pybullet_data
 from robot_bases import BodyPart
```

You need to add `pybullet_envs.` for `robot_bases` like follows.

```robot_locomotors.py
 from pybullet_envs.robot_bases import XmlBasedRobot, MJCFBasedRobot, URDFBasedRobot
 import numpy as np
 import pybullet
 import os
 import pybullet_data
 from pybullet_envs.robot_bases import BodyPart
```