Studies on the bionomics of adult mosquitoes were carried out in the Prado Basin of southern California during 1985-86. The faunal composition of mosquitoes caught by species was (in descending order) Culex quinquefasciatus, Cx. tarsalis, Cx. erythrothorax, Cx. stigmatosoma (formerly Culex peus) followed by Anopheles freeborni, Culiseta particeps, Cs. inornata and Cs. incidens. The number of mosquitoes per trap night was the lowest during December through February, and the highest during August through October. Depending on both intrinsic and extrinsic factors, adult mosquitoes were active at dusk and dawn. In spatial distribution studies, both adult and larval collections showed that Cx. quinquefasciatus and Cx. stigmatosoma were associated with dairy lagoons and Cx. erythrothorax with duck ponds and a nearby wooded area. Culex tarsalis was found in greater number at all habitats. Anopheles freeborni and Culiseta spp. were found around the wooded area. In vertical distribution studies, more mosquitoes were captured at the highest (6 m) level than at lower (0.6 and 3 m) levels which was probably due to the large percentage of parous females present at this site.