There are two dose-dependent barriers to the transmission of western equine encephalomyelitis virus in infected female Culex tarsalis. In the first, virus multiplies in the mesenteron, but does not invade other tissues regardless of the length of extrinsic incubation. We call this the “mesenteronal escape” barrier. In some mosquitoes virus escapes from the infected mesenteron but a second barrier prevents infection of the salivary glands and perhaps neural tissues. We designate this the “salivary gland infection” barrier. The effectiveness of the second barrier decreases with time but still is evident after 21 days of extrinsic incubation. The distribution of virus in the tissues of nontransmitting females with either of these barriers is described.