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Midguts of two strains of the mosquito vector Culex tarsalis were examined by light and electron microscopy following infection with western equine encephalomyelitis virus. Infection of the highly susceptible Knight's Landing strain with high-titered blood meals resulted in pathologic changes in the midgut epithelium after 2–4 days of incubation; lesions included sloughing of epithelial cells into the lumen and necrosis of cells in situ. Infection of Knight's Landing strain mosquitoes with low-titered blood meals and infection of the less susceptible Fort Collins strain with high-titered blood meals did not result in a significant increase in detached luminal cells, with respect to uninfected controls. Sloughing of infected cells into the midgut lumen may contribute to modulation of the mosquito infection. Lesions in the midgut of Cx. tarsalis are inconsistent with traditional views that regarded arbovirus infections of mosquito vectors as non-pathologic. These findings demonstrate that mosquito pathology is not an oddity limited to the previously described interaction between Culiseta melanura and eastern equine encephalomyelitis virus, and suggest that alphaviruses in general may adversely affect their mosquito vectors in nature.