Members of the Culex pipiens Linn. complex in the eastern, southern, and central United States are the primary vectors of St. Louis encephalitis virus. Although species and subspecies in the complex can be identified as 4th-instar larvae and by characters on the male genitalia, adult females cannot be identified accurately. In this study a ribosomal DNA (rDNA) segment that includes the internal transcribed spacer region (ITS) was amplified from Culex pipiens pipiens Linn., Culex quinquefasciatus Say, and Culex restuans Theobald. The DNA was amplified from single abdomens or single legs. The amplified rDNA segment from Cx. restuans is 90 base pairs smaller than those from members of the Cx. pipiens complex. Ribosomal DNA was amplified separately from 3 individuals for each population of Cx. pipiens and analyzed by restriction digestion. Intrapopulation variation is seen, because for each population, bands are present that are common to all 3 individuals within the population, but are also unique to that population. These results indicate that this method may provide a means for distinguishing among the mosquitoes in the Cx. pipiens complex.