Great American cockroaches, colloquially called "radroaches" or simply "giant cockroaches", are giant versions of the pre-War species that have increased in size presumably due to mutation caused by the nuclear fallout of The Great War.
Characteristics
Biology
Radroaches are about the size of cats with a shiny brown carapace. Unlike their modern-day cousins, they don't mind the sunlight and will often be encountered in well lit areas, such as out in the open near the Tepid Sewers. They are very territorial organisms and will attack most, if not all, things that pass close by. This often leads to their death as they are not very durable adversaries.
Gameplay attributes
Radroaches are low-level enemies, hostile to the player. They are usually encountered in groups of 5-7. They "pounce" at their enemies and bite/pinch them with their small pincers while making a screeching sound.
Statistics
Radroaches all share the same vital statistics, but can differ in aggressiveness and confidence.
Other than in Vault 101, they are found throughout the wasteland, with the most notable locations being the Nuka-Cola plant offices and the Gillian house in Minefield. The RobCo Facility has the highest number of radroaches, 31 in total.
Radroaches are one of the weakest creatures to come across in the wasteland, about on par with bloatflies for being the weakest enemies in the game.
Although most radroaches appear hostile, there are certain circumstances where you may find them as passive.
Radroaches do not give out experience points unless the Lone Wanderer is playing on normal difficulty or higher, but even then it's only one experience point.
Using Unarmed in V.A.T.S. against a radroach will result in the player stomping on it rather than punching it.
The radroach is the first type of hostile creature the Lone Wanderer will encounter in Fallout 3, inside Vault 101 when the he/she needs to kill it during the quest Growing Up Fast with a BB gun.
There are two unique radroaches in the game, Fluffy and Jitters, both imprisoned in a cage in the sniper shack.
Likewise, there are two undersized caged radroaches kept as pets by Gallo at his quarters within County sewer mainline.
During the quest Escape! you have an option to help Butch DeLoria by killing 3 radroaches to save his mom for some positive Karma.
When radroaches detect you, they won't be hostile at first but if you stand too close for too long then they will attack.
For the most part, radroaches are the only enemy in the game that won't attack on sight.