This paper describes genetic variation within and between temporal and spatial population samplings of Tursiops truncatus in the Mississippi Sound. Samplings were made from Mobil Bay, Alabama to Biloxi and Cat Island, Mississippi and at these sites over time. Five polymorphic RBC enzyme systems were analyzed. There were statistically significant differences in allele frequencies and in genotype distribution of these alleles both between sampling locations and between different samplings in the same area overtime. Individuals from certain sampling times showed striking genetic homogeneity for these protein loci as compared with other samplings, raising the question of whether these animals represent a homogeneous local group of dolphins or whether they are representative of a larger Tursiops population with fixed alleles whose major range extends into the Gulf of Mexico and who periodically move into the Sound area. Comparisons are drawn with Tursiops, social units of the Sarasota, Florida areas supporting the latter hypothesis.