This environment is part of the Atari environments. Please read that page first for general information.

Action Space


Observation Space

(210, 160, 3)

Observation High


Observation Low





You are stuck in a maze-like space ship with three aliens. You goal is to destroy their eggs that are scattered all over the ship while simultaneously avoiding the aliens (they are trying to kill you). You have a flamethrower that can help you turn them away in tricky situations. Moreover, you can occasionally collect a power-up (pulsar) that gives you the temporary ability to kill aliens. Detailed documentation can be found on the AtariAge page.


By default, all actions that can be performed on an Atari 2600 are available in this environment. Even if you use v0 or v4 or specify full_action_space=False during initialization, all actions will be available in the default flavor.


By default, the environment returns the RGB image that is displayed to human players as an observation. However, it is possible to observe

  • The 128 Bytes of RAM of the console

  • A grayscale image

instead. The respective observation spaces are

  • Box([0 ... 0], [255 ... 255], (128,), uint8)

  • Box([[0 ... 0] ... [0  ... 0]], [[255 ... 255] ... [255  ... 255]], (250, 160), uint8)

respectively. The general article on Atari environments outlines different ways to instantiate corresponding environments via gym.make.


You score points by destroying eggs, killing aliens, using pulsars, and collecting special prizes. When you are caught by an alien, you will lose one of your lives. The number of lives you have depends on the game flavor. For a table of scores corresponding to the different achievements, consult the AtariAge page.


env = gym.make("ALE/Alien-v5")

The various ways to configure the environment are described in detail in the article on Atari environments. It is possible to specify various flavors of the environment via the keyword arguments difficulty and mode. A flavor is a combination of a game mode and a difficulty setting.


Valid Modes

Valid Difficulties

Default Mode


[0, ..., 3]

[0, ..., 3]


You may use the suffix “-ram” to switch to the RAM observation space. In v0 and v4, the suffixes “Deterministic” and “NoFrameskip” are available. These are no longer supported in v5. In order to obtain equivalent behavior, pass keyword arguments to gym.make as outlined in the general article on Atari environments. The versions v0 and v4 are not contained in the “ALE” namespace. I.e. they are instantiated via gym.make("Alien-v0").

Version History#

A thorough discussion of the intricate differences between the versions and configurations can be found in the general article on Atari environments.

  • v5: Stickiness was added back and stochastic frameskipping was removed. The entire action space is used by default. The environments are now in the “ALE” namespace.

  • v4: Stickiness of actions was removed

  • v0: Initial versions release (1.0.0)