Daily periodicity of preovipositional behavior and egg deposition by the onion fly,Delia antiqua(Meigen), was monitored hourly under laboratory conditions with a photoperiod of 16:8 (L:D) and a female density of 30 per 1,260 cm3. Although preovipositional behaviors occurred in appreciable frequency throughout a day, most eggs were laid 10–12 h after lights on. When flies were tested in a chamber with photoperiod shifted 6 h from the solar photoperiod, periodicity in egg deposition remained strictly entrained with chamber photoperiod. This ovipositional periodicity corresponds well with the afternoon peak of catches in flight traps in the field. These results suggest that behavioral observations in the field and short-term control measures (e.g., volatile insecticides) directed against ovipositing flies may be performed best during the late afternoon