A polymerase chain reaction assay that detects differences in the 2nd internal transcribed spacer of ribosomal DNA was tested as a means to identify all immature stages of 2 species in the Anopheles quadrimaculatus sibling species complex. The technique was successful in amplifying DNA from eggs, 1st-4th instars, and pupae of An. quadrilmaculatus Say and An. inundatus Reinert.