Resting Culex nigripalpus Theobald were collected with a ground aspirator three times a week in Indian River County, Fla., from January 1985 through December 1987. The number of green (newly emerged), empty, blood-fed, and gravid females in each collection was counted; up to 100 empty females were dissected to estimate the abundance of empty, parous females in the population. In general, this species was uncommon during the dry season from January through June but increased during the wet season in July and remained abundant through early winter. During the wet season, green, parous, and gravid females were periodically abundant. Time series analysis demonstrated that green and parous mosquitoes were positively associated through time and that gravid females were associated negatively with daily rainfall. The strongest correlations were recorded in 1985, a year during which St. Louis encephalitis virus was detected with increased frequency in Indian River County. Laboratory experiments in a large outdoor wind tunnel confirmed that high relative humidity was important for the initiation and successful completion of oviposition by gravid, field-collected Cx. nigripalpus mosquitoes. Thus, intermittent rainfall during autumn may serve to delay oviposition, increase longevity, synchronize blood feeding, and enhance transmission of St. Louis encephalitis virus by infected mosquitoes in the field.