Diapause in Wyeomyia smithii (Coquillett) was shown to be a function of photoperiod, and independent of temperature. The critical daylength for a population from Pinawa, Man., was 15 hr light per diem. The photoperiodic cues are monitored by the early instars, with diapause being expressed in the third instar. Development is also limited by temperatures below 15 °C even when the critical daylength is exceeded.Survival at low and subzero temperatures is aided by the diapause state. However, larvae are unable to withstand extended periods of subzero temperatures, even in the diapause condition. At −5 °C, 60% mortality occurred after 8 weeks under laboratory conditions. In the field, where ground temperatures averaged −3.7 °C during the five coldest months, larval mortality averaged 45% after four winter months.Once diapause is established, larvae consume very little food. Diapause larvae appear to be as active as non-diapause larvae, and also appear to be feeding constantly. However, the amount of food ingested is negligible compared to non-diapause larvae.