The distribution of western equine encephalomyelitis (WEE) virus in the alimentary tract of Culex tarsalis (Coquillett), following blood feeding, was examined using autoradiography. Following engorgement on chicks circulating radiolabeled virus, WEE virus in all mosquitoes was concentrated at the margins of the abdominal midgut, adjacent to the epithelium. Thirty percent of mosquitoes also contained small amounts of virus in the foregut, ventral diverticulum, and/or thoracic midgut. Mosquitoes imbibing labeled virus in hanging drops containing washed blood cells and sucrose showed an altered distribution of WEE virus. Virus was not concentrated adjacent to the abdominal midgut epithelium, and all mosquitoes contained large amounts of WEE virus in most regions of the thoracic alimentary tract. These findings may help explain the increased thresholds of infection associated with artificial feeding techniques and indicate that artificial blood meals may alter the sites of initial arbovirus infection within the alimentary tract of mosquito vectors.