Radroach

Giant cockroaches are obviously giant versions of the regular cockroaches mutated by radiation - there was never any doubt that they would survive a nuclear war. Found mainly in underground areas and sewers, these creatures can usually be found in fairly large groups. They feed on the dead, but will go after living creatures if isolated. Their numbers are usually their greatest advantage. Giant cockroaches live mostly in the areas east of California. It is said that cockroach guts are actually useful in the treatment of radiation poisoning, and are widely used by tribals for that purpose.

Variations
There are several variations of giant cockroaches - some bigger and some smaller. There have even been rumors of sightings of cockroaches as big as cars.

In Texas a variation called a "flying roach" can be found. When attacking, they leap through the air and claw or bite a targeted creature - dodge quickly to avoid these flying pests, as they are very mobile.

Appearances in games
Giant cockroaches appear in Fallout Tactics and Fallout: Brotherhood of Steel. They were also mentioned in Fallout Bible 1.