 Egg | It's not really an enemy, but it does release a facehugger if it opens up. If you destroy the egg before it releases the Alien inside, you also destroy the facehugger. |
 Facehugger | The facehugger comes out of an egg. It leaps toward your face, and that is its only attack. |
 Chestburster | Chestbursters are annoying because you can only hurt them with the plasma caster/gun or when they jump towards you. They usually come out of human host bodies along the wall or ground, or come out of Infectoids. They jump towards you and latch onto your arm. |
 Warrior Alien | These are your basic Aliens. They usually attack you in large numbers. They usually just claw at you, but will also use their tail on you or bite you if you don't attack them. Some Warrior Aliens also spit acid. |
 Stalker Alien | This type of Alien has less health than the Warrior Alien. At a distance it will remain on all fours, or jump towards you. When you are near a Stalker Alien, it gets up onto two feet. |
 Arachnoid Alien | The Arachnoid Alien crawls along the ground like a spider. It stands up when it comes very near to you to attack and usually attacks in groups. It can also jump at you. |
 Smasher Alien | These Aliens are similar to Warrior Aliens except they have more health and a unique style of attacking. They lower their heads and try to headbutt you. You can strike them while their heads are lowered to disrupt the attack. |
 Defender Alien | Defender Aliens are very similar to Stalker Aliens, except they have a very high defense. They can put up their arms and defend themselves against most of your attacks, but if you use a combo on them, the last attack will hit. |
 Royal Guard Alien | This Alien is almost like a boss. The praetorian has many attacks besides the basic claw attack. It can spray a shower of acid spit onto you, run at you and ram you with its head, strike you with its tail, bite you, grab you with its claws and body slam you, and grab you with its tail to hold you in the air and bite you. The Royal Guard Alien has the most health out of all Aliens you encounter, besides the bosses, and is really more like a mini boss. |
 Infectoid | An Infectoid is a human with a chestburster inside him. They move like zombies, and sometimes carry weapons, usually a knife or Pulse Rifle. They can grab a hold of you and the chestburster inside will come out and bite you repeatedly. Their defense is very poor, as you can kill them in a few hits; their bodies just fall apart. If you take too long in killing them, the chestbursters will hatch from them. |
 Private Marine | A human Marine that is usually equipped with a knife or Pulse Rifle. |
 Sergeant Marine | A human Marine that has more health than a Private Marine, and can also be better equipped with weapons such as a Smart Gun. |
 Corporal Marine | These Marines often have Smart Guns and have the most health out of the three types of Marines you encounter. |
 Sentry Gun | These guns are stationed in various levels and fire at you in short bursts. They cannot move, so it's easy to avoid the line of fire and get behind them to destroy them. |
 Chrysalis Alien (Boss) | Chrysalis is the very first boss you encounter. Like all bosses, Chrysalis can summon other Aliens to come and fight; he summons Warrior Aliens and Stalker Aliens. He can claw you and hit you with his long tail, and he can also curl up into a ball and roll into you if you are keeping your distance. Later on in the game, you encounter Chrysalis Aliens as regular enemies, but you only come across four. |
 Razor Claws Alien (Boss) | This Alien does not have any distinguishing special attacks really, but he is very fast. When he swipes at you with his claws, his arms are nothing but a blur. He also tends to ram you if you stay away from him. |
 Alien Queen (Boss) | You fight the Queen within her egg chamber. Because she is still attached to her egg sac, she really can't move, but don't let that fool you. Her second set of jaws can easily reach out to you and hit you. She can also claw you or strike you with her tail, spit acid on you, and summon Aliens. |
 Power Loader (Boss) | The Power Loader has many attacks. It can hit you with its large mechanical arms, shoot at you with a built-in flamethrower, grab a hold of you and use the flamethrower on you, and ram into you. It can also summon more Private Marines to fight you. |
 Mad Predator (Boss) | This can be a tough enemy to defeat. He moves very quickly, and his initial attack is to jump up on the railing, then jump down and try to strike you. Once on the ground, he can attack you and also fire his plasma caster at you. As you deal more damage to him, he begins to activate his cloak as well. |
 Alien Queen (Boss) | Bet you didn't think you'd have to fight her again! This time she has no egg sac, so she can move freely around. She has the same attacks as before, biting, clawing, and striking you with her tail, plus a few new ones. She can disappear from sight by jumping up and drop her tail down. She tries to run you through with her tail and bite you. |
