We present evidence that gene flow counteracts directional selection to maintain a high level of polymorphism at Pgm-3, a gene known to have a major effect on reproduction in the population of Solenopsis invicta that we studied. Reproductive queens in this population never possess the homozygous genotype Pgm-3(ala), whereas prereproductive and nonreproductive females possess it at substantial frequencies. The loss of Pgm-3(ala) queens, which occurs because workers selectively destroy all such queens as they initiate reproductive development, constitutes a process of strong negative selection on the allele Pgm-3(a) that now has been observed over a 4-yr period. This allele is maintained at high frequency in the study population in spite of such directional selection by means of gene flow from a population of a different social form, in which equivalent selection is not found and the allele Pgm-3(a) is common. Evidence for such gene flow comes from two sources in this study. First, Pgm-3 genotype and allele frequencies for parental and offspring generations suggest that a large majority of matings at our six study sites are between resident females and immigrant males of the alternate social form. The inferred proportions of matings attributable to immigrant males at each site vary predictably according to the distance from an upwind source of such males. Second, the proportions of queens that are mated within the study population depend to a large extent on the proximity of the queens to an upwind source of immigrant males. This system offers a unique view of how gene flow occurring in the face of strong, locally restricted selection can affect the extent and distribution of genetic variation.