The abundance and species composition of adult mosquitoes collected by carbon dioxide-baited suction traps and gravid traps in western Los Angeles County, CA, were compared before and during a strong El Niño – Southern Oscillation (ENSO) cycle from December 1996 until November 1999. Following El Niño conditions in the winter 1997–1998, adult host-seeking mosquito abundance during spring was twice that observed during spring 1997 and species composition favored cool-weather mosquitoes such as Culiseta incidens and Culex tarsalis. The comparatively cool temperatures from early April until early June and increased rainfall of the 1998 El Niño negatively affected warm-weather mosquitoes such as Culex quinquefasciatus that inhabit eutrophic habitats such as urban storm drains. Gravid mosquito abundance during the early summer following El Niño conditions also increased 2- to 3-fold relative to 1997, but gravid mosquito species composition was not significantly affected by ENSO cycles, reflecting an inherent bias of gravid traps to collect predominantly Cx. quinquefasciatus. Relative to spring 1997, host-seeking and gravid mosquito abundances were reduced 3- to 7-fold from March until June 1999 under the comparatively dry La Niña conditions. The increased abundance and prolonged hostseeking activity of Cx. tarsalis during the spring and early summer following a strong El Niño may have a significant impact on public health in urban southern California because this mosquito is an important arbovirus vector and constructed wetlands in urban areas may increase suitable, comparatively permanent developmental sites for important mosquito vectors such as Cx. tarsalis that are usually rare in urban environments.