Haematophagous Diptera landing on a horse were removed by vacuum and their numbers 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 (9 spp.), biting midges (Culicoides sonorensis), and black flies (Simulium bivittatum). Mosquitoes were predominantly collected from the head, crest, withers, neck, chest, and shoulders of the horse while biting midges and black flies were predominantly collected from the ventral midline of the horse. Culex erythrothorax was the most abundant mosquito species collected overall, with considerable variation in species composition by site. Frequency of engorgement for mosquitoes captured from the horse ranged by species from zero to 58.3% with Cx. quinquefasciatus having the lowest value (16.7% or 1 of 6 mosquitoes) of species that fed on the horse. The number of insects captured at the horse and paired CO2-baited suction trap were not different (p > 0.05) for Anopheles franciscanus, Cx. tarsalis, and S. bivittatum. Culex quinquefasciatus was captured in greater numbers in the CO2-baited suction trap (p=0.0002) while An. hermsi, Cx. erythrothorax, Culiseta inornata, and Cs. particeps were captured in greater numbers from the horse (p < 0.05). West Nile virus was detected in one pool of Cx. tarsalis. Considering the horse biting rate of each of these species and their vector competence for West Nile virus, both Cx. erythrothorax and Cx. tarsalis should be considered likely epizootic vectors of West Nile virus to horses in southern California.