Interpretation of genetic differentiation values is often problematic because of their dependence on the level of genetic variation. For example, the maximum level of G_ST is less than the average within population homozygosity so that for highly variable loci, even when no alleles are shared between subpopulations, G_ST may be low. To remedy this difficulty, a standardized measure of genetic differentiation is introduced here, one which has the same range, 0–1, for all levels of genetic variation. With this measure, the magnitude is the proportion of the maximum differentiation possible for the level of subpopulation homozygosity observed. This is particularly important for situations in which the mutation rate is of the same magnitude or higher than the rate of gene flow. The standardized measure allows comparison between loci with different levels of genetic variation, such as allozymes and microsatellite loci, or mtDNA and Y-chromosome genes, and for genetic differentiation for organisms with different effective population sizes.