A new measure of the extent of population subdivision as inferred from allele frequencies at microsatellite loci is proposed and tested with computer simulations. This measure, called R(ST), is analogous to Wright's F(ST) in representing the proportion of variation between populations. It differs in taking explicit account of the mutation process at microsatellite loci, for which a generalized stepwise mutation model appears appropriate. Simulations of subdivided populations were carried out to test the performance of R(ST) and F(ST). It was found that, under the generalized stepwise mutation model, R(ST) provides relatively unbiased estimates of migration rates and times of population divergence while F(ST) tends to show too much population similarity, particularly when migration rates are low or divergence times are long [corrected].