Genetic variation was evaluated in populations of the eastern oyster Crassostrea virginica (Gmelin) from the coast of Veracruz, eastern Gulf of Mexico. We sampled six lagoons and analyzed variation at five microsatellite loci. Significant Hardy-Weinberg deviations occurred at all loci and were attributed to the presence of null alleles. We found no isolation by distance among the populations in these lagoons, but significant heterogeneity was observed among some adjacent lagoons, possibly reflections of geographical factors and local reductions in population size. Certain extreme north and south localities were not genetically different in terms of nonsignificant pairwise F_ST values. Gene flow is attributed to seasonal shifts in coastal currents, larval production throughout the year, human interference with the natural lagoon processes, and restocking efforts. The observed pattern of variation could be another example of chaotic genetic patchiness in marine organisms.