The effect of temperature on the rate of adult eclosion, blood digestion, and ovarian development and the effect of temperature and photoperiod on ovarian follicle size in unfed females was studied in colonized strains of 2 North American mosquito species: Culex restuans Theobald and Cx. salinarius Coquillett. At 20 and 25 C the processes studied proceeded at about the same rate. At lower temperatures, however, rates were significantly slower in Cx. salinarius. Both the temperature and photoperiod to which females were subjected from the pupal stage until 6-8 days post-eclosion influenced the development of ovaries in Cx. restuans: a combination of 15 C and a photophase of 8/24 hours resulted in the ovaries remaining in a diapause condition. In Cx. salinarius, photoperiod had no effect on ovarian development and ovaries did not undergo diapause under any treatment combination, although the rate of development was slower at lower temperatures so that 15 C follicles were smaller when measured at 6-8 days post-eclosion. The ecological significance of these findings is discussed.