for the gene products of 40 protein-coding loci using starch gel electrophoresis. A nested contingency-table analysis, a cluster analysis of genetic distances between populations and a gene-diversity analysis of allele-frequency data each showed that there was little genetic differentiation among North American populations and among European populations. These analyses also showed that the magnitude of transoceanic differentiation was not greater than that among regions on both sides of the Atlantic Ocean. The lack of transoceanic differentiation could be explained by a radiation model where a single population, which might have been restricted to one side of the Atlantic Ocean by coastal glaciation during the Pleistocene, radiated into several populations that do not now exchange genes.