Development times and survival of immatures and reproductive diapause of adult females of Culex tarsalis, Cx. restuans and Culiseta inornata were investigated from hatching to adult emergence at 15, 20 and 25 degrees C in the laboratory and at natural temperatures and photoperiods in southern Manitoba. Based on patterns of oviposition in artificial pools operated from mid-April to the end of September and development time of the immature stages, 3 generations of Cx. tarsalis, Cx. restuans and Cs. inornata were possible in 1980 and 1981. In 1981, 70% of field-reared Cx. tarsalis females emerging in mid-August were in diapause. Field-reared Cx. restuans and Cs. inornata entered reproductive diapause 2-3 weeks later than Cx. tarsalis.