The German cockroach, Blattella germanica (L.), and the closely related species B. bisignata (Brunner) belong to the germanica species group. They are similar in appearance, life history, reproductive cycle, and courtship behavior. The most significant difference is habitat preferences: B. germanica is a household species and lives in crowded conditions, whereas the feral B. bisignata lives outdoors in a solitary manner. Nevertheless, B. bisignata has recently been found in households. A comparison between the two species has shown that B. germanica displays gregarious behavior and produces an aggregation pheromone, whereas both characters are absent in B. bisignata. Mate preference experiments have revealed that B. germanica females accepted only conspecific males, whereas B. bisignata females mated with males from both species, provided that long distance calling was bypassed. In addition, the high reproductive potential of B. germanica outcompeted the other species: when 10 pairs of B. germanica and of B. bisignata were kept together in crowded conditions during 3 mo, B. bisignata was driven into extinction. It is concluded that the chances of B. bisignata becoming a new household species are remote.