During the 19th and early 20th centuries, malaria was endemic in many areas of the United States. Although indigenous transmission was interrupted by the 1940s, recent outbreaks in New Jersey, New York, and Texas have underscored the potential for reintroduction of mosquitoborne transmission of malaria in the United States. This report summarizes the investigation of a case of Plasmodium vivax malaria diagnosed during September 1995 in a resident of Michigan with no history of international travel; the findings of the investigation indicated that the route of transmission was probably through the bite of a locally infected Anopheles spp. mosquito.