Culiseta melanura were tested for response to changes of photoperiod and temperature in efforts to induce larval diapause. Conditioning of larvae and the preceding maternal generation were attempted: photoperiods of 9 hours light and 15 hours dark (9:15) and 16 hours light/8 hours dark (16:8) were combined with temperatures of 28, 19, 10, and 4 C in a series of experiments to determine their effects on this mosquito. Conditioning of the maternal generation had no effect on progeny. At high temperatures (28% C) all larvae completed development within the same time interval regardless of photoperiod. At 19 C pupation rates differed: nearly all of the larvae reared under the long (16:8) photoperiod pupated, whereas in the short photoperiod group only approximately 20-30% of the larvae became pupae within the test interval (17 weeks). Regardless of photoperiod conditioning, all larvae could be kept from pupating by low temperatures. One group of 4th instar larvae survived over 7 months at 4% C. Upon rewarming a substantial number pupated and emerged. All procedures were conducted with laboratory reared specimens.