Hematophagous Diptera landing on a horse were removed by vacuum, and their numbers were related to a paired carbon dioxide-baited suction trap at three locations in southern California where West Nile virus activity was high during the preceding year. Insects collected from the horse included mosquitoes (nine species), biting midges (Culicoides sonorensis Wirth & Jones), and black flies (Simulium bivittatum Malloch). Mosquitoes were predominantly collected from the head, crest, withers, neck, chest, and shoulders of the horse, whereas biting midges and black flies were predominantly collected from the ventral midline of the horse. Culex erythrothorax Dyar was by far the most abundant mosquito species collected overall. Frequency of engorgement for mosquitoes captured from the horse ranged by species from zero to 58.3%, with Culex pipiens quinquefasciatus Say having the lowest value (16.7% or one of six mosquitoes) of species that fed on the horse. The number of insects captured at the horse and paired CO₂-baited suction trap was not different for Anopheles franciscanus McCracken, Culex tarsalis Coquillett, and S. bivittatum. Cx. p. quinquefasciatus was captured in greater numbers in the CO₂-baited suction trap, whereas Anopheles hermsi Barr & Guptavanji, Cx. erythrothorax, Culiseta inornata (Williston), and Culiseta particeps (Adams) were captured in greater numbers from the horse. The horse biting rate was very low for Cx. p. quinquefasciatus, intermediate for Cx. tarsalis, and very high for Cx. erythrothorax. Both Cx. tarsalis and Cx. erythrothorax should be considered likely epizootic vectors of West Nile virus to horses in rural southern California.