Culex nigripalpus adults were collected from sentinel chickens at 4 sites in Indian River Co. (IRC), Florida. Chickens located in well drained, open habitats attracted fewer Cx. nigripalpus than did those located in a moist, forested area, but the number of mosquitoes collected in the open habitats increased significantly with heavy autumn rainfall. All St. Louis encephalitis virus (SLE) seroconversions (11/674) during a 7-year period were from chickens located in open, well drained habitats. A comparison of monthly rainfall during years of known SLE activity in IRC with the 22-year average showed that virus activity was significantly associated with years in which rainfall was unusually low in either September or October. Sentinel chickens in areas with large vector populations may not be the most reliable indicators of local SLE activity and autumn rainfall in September and October may influence yearly SLE patterns in southern Florida.