The distribution of the West Nile virus (WNV) in the organs and tissues of the mosquito Culex pipiens pallens, a potential vector of WNV in China, was investigated up to 14 days after oral infection. The WNV antigen was detected in paraffin-embedded mosquitoes using immunocytochemistry and viral titers of post-infected mosquitoes determined by plaque assay. Viral titers sharply decreased 24 h post-infection, were undetectable for the first few days, then rose over the course of infection. The first midgut infection appeared after one day, and the overall infection rate (based on midgut infection) was 43.9%. Other tissues, including hindgut, foregut, ovarian follicles, Malpighian tubules, and ommatidia, showed weak WNV antigens as early as three days post-infection. Staining in the salivary glands first appeared after seven days, and the salivary gland infection rate on the 14th day was 37.5%. Specimens with no detectable WNV antigens in any tissues, and with positive results confined to the midgut, anterior midgut, and hindgut, were observed on the 14th day. The route of viral dissemination from the midgut, and the relative importance of amplifying tissues in mosquitoes' susceptibility to infection, were evaluated. The results indicate that Cx. p. pallens has the ability to harbor WNV throughout its alimentary system and that midgut epithelial cells may be the initial site of the replication of this virus in this species.