Aliens are diminutive, hairless, yellowish-green-skinned humanoids with short, wiry limbs and green blood. Their heads are disproportionately large, with round, heavy-lidded, solid black eyeballs with no discernible iris or pupil; a thin, toothless mouth with green tongue, and two pairs of small holes where ears and nostrils would be. Aliens have no opposable thumbs, each hand having three long, tentacle-like fingers with large suction cups at each fingertip that allow them to manipulate objects. They breathe a similar atmosphere to humans, and (at least those on Mothership Zeta) subsist mainly on a diet of strange alien worms and squid.
Most aliens encountered are either hostiles or workers. Hostile aliens sometimes have an inertia suppression field that acts as a shield and boosts their Damage Resistance significantly. In combat, they tend to hang back or rush the player character if they are using melee weapons.
Regular aliens wearing the white spacesuit are the most common alien the Lone Wanderer will see aboard Mothership Zeta. They act as the standard enemy of the area and will always be hostile towards the Lone Wanderer. Aliens have many tiers, depending on the player character's level. Tiers 1-3 do not have an inertia suppression field and the aliens tier 4-6 do. An alien that is wearing an I.S.F. has a body that shimmers and becomes slightly distorted.
The alien captain is the leader of the alien forces on mothership Zeta, equipped with a modified alien blaster and a shiny, gray spacesuit. Despite being the leader of Zeta's aliens, its stats are the same as the lowest-ranking aliens in the game, and its weakness in combat is compounded by its lack of an inertia suppression field. As the ship's captain, it has remote access to the mothership's shield and death ray power systems, and a big button in front of it that can fire the death ray.
Alien workers are the working force of the Mothership Zeta. Although displayed as hostile, they will never attack the player character, but will flee or cower in fear instead. Killing one results in a loss of 50 Karma, although this may not be the case if the player character is not directly involved, such as overloading the assembly when they are near it. During the quest Not of This World, when the Lone Wanderer first spots them, Sally will tell the player character not to hurt them because they are harmless.
Sometimes, when alien workers appear to cower in fear in front of the player, they may clip into the floor. [verified]
Alien workers can cause scripting issues throughout the game. The player's colleagues react differently if one is near, often disrupting scripts. Thus, for example, if there are alien workers in the observation bay area when the Lone Wanderer first enters, when Sally ports through, she may well port straight out again, without opening the doors. If scripting errors occur, it may be advisable to reload to a previous save and kill all the alien workers before trying to trigger a script. This can have a significant impact on your Karma, although a Karma loss can be avoided if another non-player character kills them, such as a friendly drone or abominations. [verified]
When using VATS, especially with a long VATS session, aliens with Shock Batons will constantly rush to melee distance and will be striking the player character 2 to 3 times for each 1 time the player's Gun fires. This is especially noticeable on Very Hard when fighting against Shielded Aliens. What's wrong is that the player is moving in slow motion, meaning the alien is moving two or three times faster than he should be as he scores more attacks on the player.[verified]