Ethanol extracts from the bodies of male or female American cockroaches,Periplaneta americana(L.), repel conspecifics of all ages and sexes from shelters during the photophase. Full repellency is obtained above a dosage of 1.6 cockroach equivalents per shelter. The repellent is endogenously produced, is distributed throughout the bodies of individuals of both sexes, and is effective against at least four other species of cockroaches of diverse phylogenetic relationship, including the German cockroach,Blattella germanica(L.). There was no evidence that the repellent is released by living insects as an alarm pheromone. The active materials (identified elsewhere as unsaturated fatty acids) differ from the proteinaceous dispersion-inducing substance produced by living German cockroaches. There was no evidence that the repellent reported here was released from living cockroaches, even at very high population densities. Cockroaches were repelled, however, by intact and ruptured corpses. The repellent may function as a cue for avoiding areas where other cockroaches have died. A literature review suggests that unsaturated fatty acids may elicit avoidance of dead conspecifics across wide phylogenies of invertebrates.