Mosquitoes collected from coastal, inland valley, and alpine locations in California were evaluated for their experimental vector competence for two viruses in the California serogroup (Bunyaviridae: Bunyavirus). Aedes squamiger, a coastal salt marsh mosquito, was an efficient vector of a California encephalitis (CE)-like virus isolated from its habitat (89% of the pledget-fed females became infected and 61% transmitted virus). Aedes dorsalis, a coastal mosquito, and Ae. melanimon, an inland valley mosquito, were competent vectors of prototype CE virus (98% and 100% of the pledget-fed females became infected and 56% and 30%, respectively, transmitted virus). Aedes squamiger and Ae. dorsalis transmitted both viruses vertically to one or more of 20 of their progeny. Culiseta inornata was susceptible to infection with both viruses, but 5% or less transmitted virus perorally. Alpine mosquitoes, Ae. cataphylla, Ae. increpitus, and Ae. tahoensis, became infected with both CE and CE-like viruses, but 3% or less transmitted virus. All species of mosquitoes were more efficient vectors of both viruses following intrathoracic inoculation than following pledget feeding, suggesting the presence of mesenteronal barriers.