Enemies

Four main categories of enemies are encountered:

Aggressors/Splicers
Deformed, genetically modified Rapture citizens who are now remnants of Ryan's army, the Aggressors cannot survive without ADAM due to their extensive biological modifications. Early Splicers seem to be referred to according to their weapon preference, such as "Leadhead" for those who prefer guns. They wear little or no armor, and normally roam the levels of Rapture, searching for other inhabitants to kill and steal ADAM from. As their name would suggest, they are aggressive and quick to attack, and will use their enhanced physical strength, group tactics and, sometimes, semi-biological weapons to kill all in their path. Most of the Splicers are wearing the masquerade ball masks that were worn during the 1959 New Year's Day party. (One character muses, "I wonder why they wear the masks? Maybe it's because they have a shred of humanity left, and they're ashamed of what they've become.")

Gatherers
(Little Sisters) The Little Sisters, genetically modified prepubescent girls, were originally conceived as slug-like, genetically modified sea creatures. The Gatherers, who were created by a scientist named Dr. Tenebaum as a solution to the ADAM shortage, extract the inert ADAM from the dead (which they call "angels") using retractable needles and then consume it, their bodies reprocessing it into a usable form. The BioShock team designed these characters in order to draw out an emotional response from players, who face an ethical choice between saving or murdering little girls in order to obtain much-needed resources. Nevertheless, doing so is severely difficult, due to their symbiotic relationship with the Protectors. According to lead designer Ken Levine, the Gatherers have a direct link with the main character. In a recently released audio file, Levine clarifies that the player cannot physically harm a Little Sister directly, but can "harvest" the ADAM from them (a process which they will not survive). This will be presented via audio, and a black screen. Players are also given the option to "rescue" the Gatherers, which nets only half the ADAM of a harvest. Ultimately, the player's decision vis-à-vis the Little Sisters will influence the game's ending.

Protectors
(Big Daddies, or Mr. Bubbles and Mr. B as the Gatherers call them) Mute, lumbering bio-mechanical monstrosities, created to protect the Gatherers. They are heavily armored and wield high-powered weaponry, usually including a large drill. Their armor somewhat resembles a large diving suit. Most of the inhabitants stay out of the way of the Gatherers and Protectors. They will not attack the player unless provoked and according to lead designer Paul Hellquist: "Once you mess with them, you find out why no one messes with them." In a recent trailer/commentary for BioShock, it was revealed that, despite their size and typically slow movements, they become incredibly fast once engaged, in some cases quicker than the player character. However, the player can defeat them easily enough if the correct plasmids are used, such as shooting bolts of lightning at a Big Daddy standing in a pool of water. There are so far two known types of Big Daddies; Rosies and Bouncers, Bouncers are the close combat type with different detachable arm-mounted weapons, one of which is a drill. Rosies attack from long range due to the large rivet gun they are armed with.

Security Bots
Throughout the city of Rapture, they are called via alarm by various security cameras. Security bots appear to hover and fly using the same method as helicopters and are armed with machine guns. When disturbed, the security cameras' siren will sound and an unlimited supply of security bots will pour out for a limited time. There are three ways of taking down security bots: the player can attempt to destroy them, shut off the security system by paying the money demanded to access a Bot Shutdown switch, or manually hack into their systems. Hacking bots and other mechanical devices prompts a mini-game resembling Pipe Dream. The security system can also be suborned by the player, either by "hacking" it or using the "Security Beacon" plasmid on an enemy.