Populations of the freshwater calanoid copepod Diaptomus sanguineus inhabiting three Rhode Island ponds switch from making subitaneous (immediately hatching) to diapausing eggs on different dates. From results of previous research the timing of diapause appears to correspond closely to the individual causes of seasonally harsh conditions in each pond. The results of rearing copepods from each pond in controlled laboratory environments indicate that each population possesses a unique spectrum of sensitivity to photoperiod. The responses obtained, however, fail to describe adequately either the rapidity with which the onset of diapause occurs in natural populations, or the differences in diapause timing between ponds. In initiating diapause, the copepods must respond to seasonal environmental cues other than critical photoperiod.