Either high temperature or long photoperiods effectively terminate embryonic diapause inAedes atropalpus. Diapausing eggs hatch after a 20-day exposure to 30°C, or a 90-day exposure to L:D 16:8. A high percentage of diapausing eggs will hatch even after 5 days at 30°C, provided they are 60–90 days old. High temperature acts independently of photoperiod, and eggs will hatch at short as well as long photoperiods. Light enters the egg via the anterior end, probably through the micropyle. Eggs with the anterior end hidden from the light did not respond to long photoperiod. The critical photoperiod for the induction of diapause for a population ofA. atropalpusfrom 44°N.L. was 14.25 h light/day. High and low levels of protein in the larval diet affected both the size of adults and the fecundity. Exposure of larvae and adults to different photoperiods affected egg production as well. Fecundity was greatest at L:D 16:8, whereas adult size was greatest under continuous light.