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Colonized and field-collected female Culex tarsalis, infected with St. Louis encephalitis (SLE) virus by intrathoracic inoculation or by feeding on a viremic host, transmitted virus to their F1 adult and/or larval progeny when reared at 18(±1)°C but not when reared at 27(±1)°C. The minimal infection rates (MIR) for different populations of Cx. tarsalis ranged from 1:32 to <1:250 (mean = 1:121) for larval progeny and from 1:32 to <1:1, 989 (mean = 1:1,571) for adult progeny. SLE virus also was transmitted transovarially by colonized and field-collected populations of Culex pipiens (mean MIRs = 1:340 and 1:1,815 for larval and adult progeny, respectively) and by a field population of Culex quinquefasciatus (MIR = 1:500 and <1:246 for larval and adult progeny, respectively), but not by colonized strains of Cx. quinquefasciatus and Culex peus. SLE virus was not recovered in tests on 5,522 Cx. tarsalis and 4,798 Cx. quinquefasciatus that were collected as larvae or pupae from field sites in Southern California and reared to adults at 18°C in the laboratory. Transovarial transmission of SLE virus by Aedes epactius was confirmed and extended to a closely related species, Aedes atropalpus. Efforts to demonstrate transovarial transmission of SLE virus by Aedes melanimon, Aedes sierrensis, and Aedes triseriatus were unsuccessful. Aedes dorsalis, Cx. peus, and Toxorhynchites amboinensis were equally sensitive hosts for viral isolation when inoculated with suspensions of larvae transovarially infected with SLE virus.