The characterization of photoperiodic induction of sexual rest inArmadillidium vulgarefemales (a cosmopolitan species) shows that the photoperiodic curves relating to the cessation of reproduction are representative of long-days type species. Experiments made on photophase maintenance or reduction show that females are sensitive not only to such reduction but also to its magnitude. Furthermore, the photophase value is relative and depends on previous conditions experienced by the female and there is evidence for the existence of a circannual endogenous cycle inA. vulgare.