In the southeastern United States there exists the so-called crucians complex which consists of three species of the genus Anopheles: crucians, bradleyi and georgianus. They are morphologically very similar, and may be separated with certainty only as fourth instar larvae. Their ecological requirements are somewhat similar, as they have been found together in the same breeding places; however, bradleyi is usually found in more stagnant water than crucians which prefers fresher, clearer water. A. bradleyi also may occur in brackish water. The members of this species group have until recently been classified as subspecies of the species crucians. A study was made of the salivary gland chromosomes of crucians and bradleyi (Kreutzer and Kitzmiller, in press), which indicated that in addition to the morphological similarities there existed a close chromosomal similarity between the two species. The banding patterns of the chromosome arms were not only similar at the ends of the autosomes, which is common among the members of the subgenus, but also extensive areas in the interior of the autosomes showed striking similarities. In addition the X chromosome of the two species showed distinct similarities, a phenomenon not demonstrated before in Nearctic anophelines (Kitzmiller et al., 1967). The banding patterns of the salivary chromosomes of these species differed by probably no more than five paracentric inversions, plus a few single band differences in each arm. Due to these morphological and chromosomal similarities a further study was undertaken to determine the amount of gene flow that might be possible between the two species. The method used to measure the gene flow was a series of interspecific crosses and backcrosses. The crosses made were as follows :