Radroaches are usually found in underground areas and sewers across North America, which can cause them to be a pest in certain Vaults, where they can often be found in fairly large groups. Radroaches feed on dead organisms, but they will attack living creatures if isolated. Their numbers are their greatest advantage. Radroaches live mostly in the areas east of California.
It is said that radroach innards are useful in the treatment of radiation poisoning, and are widely used by tribals for this purpose.
These radroaches are commonly found in the Capital and the Mojave Wastelands and look similar to real-life cockroaches. Radroaches were enlarged extensively by mutation: to the point that they are now as large as housecats.
They are generally little more than a nuisance to Wasteland survivors and are less than likely to pose a challenge. Radroaches are territorial and rather aggressive, but not very harmful. Anyone can kill them with their bare hands, and will only attack if you are very close; despite this, Vault 101 security personnel usually are no match for around 3 radroaches at once and will be easily disposed of. This is seen during Escape!
On one of the childhood quests in Vault 101, the Lone Wanderer, on their 10th birthday, is tasked with killing one with a BB gun given to them by Jonas Palmer as a birthday present.
In the Midwest, mutant cockroaches are a common sight in the Wasteland. As in the Capital Wasteland, they can be encountered nearly everywhere, from old buildings to plains. However, they are drastically different and more dangerous than those met in the Capital Wasteland. They have a big, protected main body and small legs, and their saliva is incredibly toxic. They are very aggressive and attack every human unlucky enough to encounter them.
The Midwestern giant cockroaches are more varied than their Capital Wasteland cousins. Their size can also vary - they can be as small as their relatives in the Capital Wasteland or as big as an adult human.
Roachor is a unique giant cockroach encountered by the Warrior of the Brotherhood of Steel. It is an "advanced" version of a large cockroach, which lived in the Peoria facility. It appears Roachor was affected by the high amount of radiation coming from a strange production device which was used in manufacturing fusion batteries. He is bigger than any large radroach—the same size as a car—and his body glows with a greenish radioactivity. His poison is deadlier than that of Midwestern radroaches.
Their inclusion in the Fallout series is probably a nod to the common myth that cockroaches could survive a nuclear apocalypse.
In real life, cockroaches are noted for their relatively high resistance to radiation—about five to ten times that of humans. However, there are other insects and arachnids even more resistant to radiation, including fruit flies and scorpions. Their resistance to radiation can be explained through their cell-cycle. A tissue-cell is most vulnerable to radiation when it divides, but the cells of a cockroach only divides once every time it molts.