This study presents nuclear DNA datasets of nine microsatellites for a new sample of Pacific Bluefin Tuna (Thunnus orientalis) from its common spawning grounds and ten loci for an additional collection of Yellowfin Tuna (T. albacares) from a locale of possible admixture in the western Pacific. Population genetic analyses of these nuclear DNA datasets support within-sample diversities that are similar to each other and to other tuna, multiple cases of heterozygote deficit or excess, and no instances of linkage disequilibrium. Bayesian estimates of effective population size are exceeded by those of census population size (as obtained from fisheries catch data) by a factor of >500. Further Bayesian inferences indicate that the individuals of the sample of T. orientalis belong to a single population, whereas those of the collection of T. albacares may derive from two or more. We now call for more critical testing of these preliminary results with additional geographic samples and automated microsatellite genotyping.