A mutant strain of the flesh fly, Sarcophaga bullata, fails to enter pupal diapause and also shows an arrhythmic pattern of adult eclosion. Pilot studies of hybridization between the mutant and wild type strains indicate a strong paternal effect on these two events. The fact that both eclosion time and diapause are altered in the mutant strain, and the fact that both are linked to male inheritance suggests a common clock mechanism that regulates circadian rhythms and photoperiodism.