Haemolymph ecdysteroid titres were determined throughout the intermoult cycle ofOniscus asellusand related to ecdysteroid-related physiological events. In non-breeding females, ecdysteroid titres rise during premoult to a peak two days prior to ecdysis in synchrony with formation of a new cuticle in the posterior half of the animal. However, the anterior half does not form cuticle during this time, but instead accumulates calcium. Thus, anterior and posterior halves of the epidermis respond differently to the same levels of ecdysteroids. The anterior half secretes a new cuticle at a later time and at a very low ecdysteroid titre. During the first breeding moult of a season, differentiation of a brood pouch commences in early premoult and the rise in ecdysteroid titre is delayed by two weeks. Both vitellogenin synthesis by the fat body and oocyte growth closely parallel the ecdysteroid titre. Although both of these processes are thought to be influenced by ecdysteroids, the titres of breeding and non-breeding females did not differ, indicating that other factors determine whether or not vitellogenesis will occur in the presence of ecdysteroids. Together, these findings show thatOniscusdisplays ecdysteroid-related events in strikingly unusual ways which raise questions concerning the regulation of cellular responses to ecdysteroids. The hepatopancreas accumulates ecdysteroids to levels well above those in the haemolymph during late premoult and appears to be the main route for their elimination from the haemolymph. Ecdysteroids in the hepatopancreas are translocated rapidly into the hindgut and discarded with the hindgut cuticle at the posterior ecdysis.