During 1994-95, totals of 17,656 adult females and 111,104 adults reared from field-collected immatures comprising 19 species in 4 genera of mosquitoes were collected from Morro Bay estuary and surrounding environs in San Luis Obispo County, California. Aedes dorsalis was the dominant summer mosquito, whereas Aedes squamiger and Ae. washinoi were abundant during winter and early spring. Host-seeking Culex tarsalis were collected infrequently, even though immatures were collected frequently from freshwater surface pools. Overall, 13,561 adults (386 pools) and 91,547 adults reared from field-collected immatures (3,027 pools) were tested for arboviruses by plaque assay in Vero cell culture. Morro Bay virus, a member of the California serogroup, was isolated from 4 pools of Ae. squamiger reared from field-collected immatures (minimum field infection rate-1.07 per 1,000), verifying the maintenance of this virus by vertical transmission. All remaining pools were negative. Three flocks of 10 sentinel chickens and one group of 5 sentinel rabbits were bled biweekly and tested for arbovirus antibodies with negative results. Neither horizontal nor vertical transmission of western equine encephalomyelitis virus was detected.