Great American cockroaches, colloquially called "radroaches" or simply "giant cockroaches," are giant versions of the most common species present before the Great War (periplaneta americana). They have increased in size due to mutation caused by nuclear fallout.
Radroaches are about the size of modern house cats and have a shiny brown carapace. Unlike their modern-day counterparts, they do not mind sunlight and will often be encountered in well-lit areas, such as out in the open near the tepid sewers. If encountered, they will first try to scare off their enemies by flapping their wings and antennae while bobbing up and down but will attack if the enemy refuses to back away. This usually ends in their death. Radroaches are one of the weakest creatures in the game, about on par with bloatflies.
Radroaches are low-level enemies, hostile to the Lone Wanderer and any companions they may have. They are also usually encountered in groups of up to seven and "pounce" at their enemies and bite/pinch them with their small pincers while making a screeching sound. They are commonly used as food for people in the wasteland and, as such, their meat can be bought from most merchants who sell food.
Radroaches all share the same vital statistics, but can differ in aggressiveness and confidence.
Radroaches are the first hostile creatures the player encounters in the game and the only hostiles that have to be killed while still in Vault 101. They can be found nearly anywhere 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, with 31 in total.
Although most radroaches appear hostile, there are certain circumstances where they may be passive, such as the roaches in the vicinity of the Roach King.
Radroaches do not give out Experience Points unless one is playing on normal difficulty or higher, but even then it's only one experience point for each roach.
Using Unarmed attacks in V.A.T.S. against a radroach will result in the player stomping on it rather than punching it, the only time this animation is ever used.
The radroach is the first type of hostile creature the Lone Wanderer will encounter in Fallout 3, inside Vault 101 when they need to kill one with a BB gun during the quest Growing Up Fast. It is also the only enemy type the Lone Wanderer needs to kill in order to complete the game.
Two undersized radroaches can be found in Vault 108's living quarters cafeteria accompanied by a regular radroach, one the size of Gallo's pets and another even smaller one.