The riverine ecology of Culex tarsalis Coq., Cx. pipiens quinquefasciatus Say and Cx. erythrothorax Dyar was compared in a focus of WEE and SLE virus transmission during a study at the New River, Imperial Valley, California in 1978. Cx. p. quinquefasciatus and Cx. erythrothorax breeding was widespread in the inundated reed/salt cedar habitat, and larvae were most frequently found alone or associated in shaded, eutrophic sites. Larval Cx. tarsalis were less prevalent, occurring solitarily in sites exposed to full sun and commonly cohabitating with Cx. p. quinquefasciatus. Larval densities of the 3 species averaged less than 5 per dip during each month of the December to May sampling season. Adult Cx. tarsalis and Cx. p. quinquefasciatus exhibited similar tendency to disperse from the breeding area as determined by tropism to CO2 -baited light traps set in a transect across the river basin. Cx. erythrothorax showed minimal dispersion from the breeding habitat. Bi-modal seasonal peaks of Cx. tarsalis density occurred in June and August correlating with minimum WEE and SLE infection rates of 39.5% and 29.1% pools positive, respectively. Infected Cx. tarsalis were recovered throughout the basin transect and at rural ranch sites 0.2-1.6 km from the breeding site. WEE and SLE viruses were not isolated from Cx. p. quinquefasciatus pools, and abundance was asynchronous with viral activity.