ABSTRACT.In mixed‐age cultures of the flesh‐fly,Sarcophaga argyrostoma(Robineau‐Desvoidy), the initiation of larval wandering (exodus behaviour) occurs as a gated circadian rhythm. In light‐dark (LD) cycles, most of this activity occurs in the dark, except in very short nights, or in certain phase relationships between the rhythm and light cycle. When transferred from series of LD cycles into continuous darkness (DD), cultures show a weakly persistent free‐running rhythm with a period of about 21 h. However, after transfer of first instar larvae from continuous light (LL) to DD, no such rhythm is observed. In contrast to larval exodus, formation of the puparia occurs at any stage of the LD cycle. The physiological mechanisms underlying this gated exodus behaviour, and its possible selective advantages, are