As part of a larger study of Culex pipiens in Memphis (Shelby County), Tennessee, egg rafts were collected from predetermined, widely distributed sites during the 1979 and 1980 breeding seasons. Marked differences in populations of related species and subspecies were observed between the two seasons, with the 1980 pattern more closely resembling that seen in 1975, when Memphis experienced epidemic St. Louis encephalitis (SLE). It is suggested that because of the observed interseasonal variability in numbers of potential vectors, Culex species and subspecies populations should be monitored until detailed comparison of epidemic and nonepidemic years becomes possible. The novel methods used herein of monitoring populations by rearing egg-raft collections from specific sites, and measuring siphonal indices of the larvae would be especially cost effective.