Under alternating light and dark, pupation in Aedes taeniorhynchus (Wiedemann) follows a diurnal rhythm with a period of about 22.2 hr, which may be truly circadian. The period of pupation rhythm is unaffected by temperature but may be lengthened by food deficiency, which prolongs development and may enhance the sharpness of the daily peaks. The emergence rhythm is dependent on the pupation rhythm, separated by an interval affected by temperature but not by photoperiod. Daily pupation episodes come to a peak between noon and midnight at a time set by temperature, diet, and time of hatching. An age threshold at each temperature can prevent the early part of the first pupation episode from expression, resulting in a small “advance peak” late in the day, separated from the next day's peak by less than the 22.2-hr regular period; these advance peaks average 83% males, and can comprise as much as ⅔ of an entire brood's males. The age threshold for pupation, set by the temperature, is nearly the same for both sexes. The 95% limits for a daily pupation increase from 8 hr at 32° to 21 hr at 22°, which indicates a breakdown in the rhythm at the colder temperatures. At full rations, the mean developmental retardation of the female behind the male increases from 5.3 hr at 32° to 14.5 hr at 22°, but only under conditions of food deficiency does it exceed a day. Larval developmental rate is directly related to temperature and amount of food available; it is lower in continuous light than in either continuous dark or alternating 12-hr periods of light and dark. The net effect of the diurnal rhythm of pupation is to reestablish for each experimental hatch group a synchrony of development lost to a remarkable degree in continuous light or dark.