The existence of a form of "parental influence" has been established in two species of Asellus: aquaticus and coxalis. The presence of adults of the same species in the breeding jars of the young individuals halves the mortality; for the young coxalis it doubles the rate of development. The hypothesis is advanced that in the two biotopes studied the species coxalis is asserting itself to the detriment of aquaticus, on account of the greater importance that the "parental influence" has in this species.