Aliens are considered the most dangerous hostile NPCs in the game. Most of the time, aliens could be attracted by the tendency which will track Bob, unlike other guards and other NPCs. Aliens don't use doors but they can teleport to another room. If an alien spots Bob, it will rush towards him quickly then attempts to abduct him. Right when the alien is abducting Bob, escaping is often useless, as the player will not run away and have no choice but to receive a game over. Although, eliminating NPCs and objects that are blocking Bob's way are the only advantages to the player if the player is careful. Take note that they are not attracted to noises, this makes the Wind-up toy and the Noisemaker useless. The Toxic Donut has no effect to aliens, except the green gas around them is still visible. The player can use the Invisibility Potion to pass through or the Teleportation Mine to make them disappear completely. Be careful when using them, otherwise, they may still ruin a perfect or lead the player to a game over.
- If an alien is tracking Bob while Bob is outside (like where the player starts in a level), the alien can actually abduct Manny Roswell.
- There is another kind of alien in one of the Seagull Bay level as an easter egg, the said alien just walks out of and into the shadows. This alien has white skin and black clothes.
- The said alien can also be seen driving the U.F.O in the level selection map of Seagull Bay.