The adult eclosion in the onion fly, Delia antiqua（Diptera: Anthomyiidae）synchronized with the environmental cycles and the time of emergence was determined as the stage of pupae. The peak of adult eclosion were observed before and after light-on under 4L:20D～16L:8D. With photophases of 18h or longer, it was observed at 4h～8h after light-on. With photophases of 10h or shorter, the emergence occrred 10～16 ｈ after light-off when observed by an infrared-light emitter and a detector（GT-1, Takenaka Electronic Industrial Co. Ltd.). When pupae were exposed to thermoperiod（warm phase: cool phase, each 12h), the adult eclosion was observed at around temperature-rise in continuous darkness（DD）and continuous light （LL）. The peak of adult eclosion occurred 10-14h after temperature-fall under warm phases of 6h or 12h and 2-6h after temperature-fall under warm phase of 18h.
The peaks of adult eclosion were repeated at about 24h interval except transfer to LL when pupae were transferred from cyclical conditions（photoperiod or thermoperiod）to constant external conditions, suggesting the involvement of a circadian time-measuring mechanism phase-set by photoperiodic and thermoperiodic signals. When pupae transferred from LL to DD（single signal of light-off), the peak of emergence appeared every about 24h. It suggested temperature compensation that the emergence rhythm was maintained with the cycle of about 24h irrespective of temperature.
The adult eclosion was observed under conditions where a photoperiod（12L:12D）and a thermoperiod （25℃:20℃, each 12h）were combined in various phase angles. It was thought that the emergence time was strongly infl uenced of thermoperiod, however, their interacting effect was observed under the treatments with an un-naturally large phase angle between thermoperiod and photoperiod.