Abstract It has been nearly 15 years since the genetics of Mytilus was the subject of a comprehensive review. In this period, our understanding of the nature of genetic variation in this group has been substantially altered. Studies on the extent of individual variation have demonstrated that genotype (i.e., multiple locus heterozygosity) has a significant effect on energy metabolism, producing significant statistical correlations between genotype and measures of both metabolic energy demand and productivity, particularly growth rate. These correlations can be mitigated by a number of factors, including age, reproductive state, the specific genes under study and ecological conditions. Studies of within-population variation have repeatedly noted marked deficiencies of heterozygotes, but no satisfactory explanation for this observation has yet been identified. Deficiencies are locus-specific and multilocus disequilibria have been described, suggesting a combination of larval mixing with additional forces such as selection, aneuploidy or molecular imprinting. Early work suggested substantial genetic differentiation between spatially proximate populations. Populations are now known to be relatively homogeneous, even over vast geographical distances. Genetic differences between proximate populations generally represent taxonomic differentiation among mytiliid species, not population differences. Studies of allozymes, morphology and (to some degree) mitochondrial genotype among world-wide samples have demonstrated the existence and geographic distributions of four species in the genus: Mytilus californianus, M. edulis, M. trossulus and M. galloprovincialis. Data for the three latter species are presented along with the geographic distribution of each.