1. Unchilled, diapausing larvae of Toxorhynchites rutilus rely on photoperiod for the maintenance of diapause. The photoperiodic clock is temperature-compensated between 16.5° and 25° C, maintaining both a similar set-joint and inherent accuracy over this range. The rates of development among larvae terminating diapause are dependent upon both temperature and photoperiod. 2. Chilling of dormant Toxorhynchites rutilus can promote response to progressively shorter daylengths, thus decreasing the critical photoperiod. Chilling can also accelerate response to long days, thereby decreasing the depth of diapause and, after prolonged exposure, can eventually terminate diapause directly, leaving subsequent morphogenesis independent of photoperiod. 3. The optimal temperature for these effects of chilling is above 4° C, below 16.5° C, and may lie around 7° C. 4. Temperatures between 5° and 15° C are vernal and autumnal rather than hibernal. The interaction between chilling and photoperiod may then represent an adaptive compromise between selection due to long-term climatic trends and the vagaries of spring weather.