Culex nigripalpus (Theobald) is a proven vector of St. Louis encephalitis virus (SLE) in southern Florida. It is an important vector for three reasons: its opportunistic blood-feeding behavior, its ability to forage and locate hosts in a variety of habitats, and its ability to retain eggs for long periods when suitable oviposition sites are unavailable. Results of laboratory and field observations documenting that Cx. nigripalpus fits the above criteria are presented. Field data are also presented showing that SLE activity in sentinel birds as well as in humans occurs in Florida during years when heavy autumn rainfalls are separated by periods of drought. Blood feeding and oviposition behaviors of Cx. nigripalpus are affected by rainfall. Large amounts of rainfall allow mosquitoes to oviposit and then to enter and blood feed in habitats where they could not previously forage because of the dry conditions. Thus, rainfall patterns are an important part of the SLE transmission cycle in southern Florida.