An electrophoretic survey of genetic variation in mosquitofish populations of the Altamaha and Ogeechee drainages in Georgia revealed significant divergence (fs,=O.270) among populations within the Altamaha. Cluster analysis showed two lineages: one formed by populations from the Ogeechee drainage and the eastern and central Altamaha branches and another consisting of populations from the Ocmulgee River, the westernmost branch of the Altamaha drainage. These lineages may represent two independent forms. Average modified Rogers’ genetic distance was 0.25% between the two groups. The O_cmulgee lineage had significantly higher multilocus heterozygosity (H=0.206) than the other one (I-/=0.120). The high heterozygosity in the Ocmulgee lineage is consistent with the hypothesis that it originated by hybridization. Populations in the Altamaha and Ogeechee exhibit spatial patterns of genetic characteristics similar to those previously described for populations in other drainages
