Phenotypes at 28 enzyme loci were analyzed by vertical starch gel electrophoresis for each of 90 populations of largemouth bass Micropterus salmoides. Allele frequencies at each locus, as well as the mean number of alleles at each locus, the average number of polymorphic loci, and the mean level of heterozygosity, were calculated for each population. Matrices of genetic identity and distance were used to assess interpopulational relationships. These analyses clearly reveal substantial genetic differences among populations in the United States. The northern subspecies M. s. salmoides and the Florida subspecies M. s. floridanus have fixed allelic differences at two loci, isocitrate dehydrogenase-B and aspartate aminotransferase-B, that can be used to determine contributions of each subspecies to the gene pool of any population. The intergrade zone between the subspecies consists of northern Florida, Mississippi, Alabama, Georgia, South Carolina, North Carolina, Virginia, and Maryland, as well as Texas, California, and perhaps a few other states in which largemouth bass with at least some of the genes of the Florida subspecies have been introduced. This increased intergrade zone is larger than previously proposed, and casts doubt upon the genetic identity of largemouth bass used in several studies of subspecific physiological properties and stocking success. Geographic distributions of the alleles encoded at four loci describe distinct latitudinal clines. This suggests a genetic influence on thermal tolerances and preferences of largemouth bass. This study demonstrates the need for incorporating genetic information and principles into current and future fisheries management programs.