The seasonal abundance of adult Culiseta inornata was markedly bimodal in the San Joaquin Valley of Kern County, California, with minima observed during both summer and midwinter. Larvae were abundant in most surface water habitats during winter, but could not be found during summer. The occasional collection of females during summer indicated the persistence of an adult population. The midwinter decrease in adult abundance was attributed to the progressive mortality of the autumnal cohort and delayed emergence due to cold water temperature. Reproductive diapause was not induced experimentally when field or laboratory populations were exposed as larvae, pupae or adults to simulated summer or winter photoperiod and temperature regimens. In comparison, Culex tarsalis readily entered a winter diapause when concurrently exposed to simulated winter conditions. The aestivation, and perhaps hibernation, of reproductively quiescent females makes Cs. inornata theoretically attractive as a maintenance host of encephalitis viruses, while the bimodal seasonality of host-seeking activity defines periods when Jamestown Canyon virus may be transmitted horizontally.