Abstract The photoperiodic control of reproductive diapause was examined in Drosophila triauraria. Experiments using non-24 h light-dark cycles revealed that the duration of scotophase plays an essential role for the determination of diapause, and night interruption experiments suggested that the process measuring scotophase was divided into two phases, A and B. The A phase lasted 6 or 7 h from the onset of scotophase, and the light pulse in this phase is assumed to reset the clock. The B phase followed the A phase, and the light pulse in the B phase is assumed sometimes to stop the clock and sometimes to have no effect (i.e. the light pulse seems to be sometimes disregarded). In the A phase, a short light pulse seems to be somewhat less effective to reset the clock than a long one. In the B phase, a short light pulse was disregarded more often than a long one. The relation between these two phases is not known, but substances or stimuli which operate the machinery of the B phase may be accumulated in the A phase, and substances or stimuli responsible for the induction of diapause may be accumulated during the B phase.