These mutated Great American cockroaches, colloquially called "radroaches" or simply "giant cockroaches," are enlarged versions of the pre-War species that have increased in size, presumably due to atomic fallout caused by the Great War.
Unlike in the previous games, the player character suffers from slight radiation damage when hit by a radroach.
Radroaches can sometimes be found infesting seemingly empty rooms, hiding in and behind desks or other furniture as well as on walls or ceilings. They will emerge and attack when the player character enters close proximity. Sometimes glowing radroaches are stuck under the flooring usually characterized by a green glow on the floor seemingly from nothing. The radroach can be dislodged if the player character uses a melee attack on the glowing part of the ground.
Radroaches come in many different physical sizes, large and small, but this does not seem to relate in any way to their toughness -- one will encounter small tough radroaches and large weak radroaches, and vice versa. Radroaches are also sometimes found to have considerable extra health compared to other radroaches of the same level, even at level 1; when it happens, this extra health does not seem to correlate in any way to their physical size, appearance, or the resistances displayed in V.A.T.S.
Similar to glowing ones, glowing radroaches have a distinctive glow, even after death, and inflict more radiation damage than their basic variant. They are often encountered among regular radroaches and are generally larger, but do generally not pose much more of a threat and can easily be disposed of.