Six British species of the littoral prosobranch Littorina have been examined for genetic variability at between eight and twenty-five enzyme loci. The characteristics and levels of variability observed are similar to those observed in other taxa of similar somatic complexity. Attempts to relate patterns of variability with enzynle structure and function, or with bionomic strategies have shown no significant trends. A phylogenetic tree has been constructed and divergence times estimated for "five members of the genus. The advantages of the littoral environment, coupled with a poly-specific genus for examining models of evolution, has been discussed. L. rudis and L. littorea have been examined in detail at several sites along the coast of South Wales and variability in each species has been examined in terms of the physical separation and the ecological properties of the populations. Whilst L. littorea is homogeneous and probably effectively panmictic in South \vales, L. rudis shows a significant increase heterogeneity as populations become further separated, indicating substantial population isolation. A highly significant relationship between heterozygosity (gene-diversity) and species overlap has been found, neither of these characteristics could be related to exposure. In populations sampled at more than one tide level, the sample from the higher mark invariably showed the higher heterozygosity. Little relationship could be found between environmental parameters or heterozygosity and measures of morphological variation, however it was found that ~ average older L. rudis and L. littorea were more heterozygous than younger individuals. Heterozygotes show a lower variance than homozygotes for a meristic character, shell index, at some loci. The data havebeen examined for linkage disequilibrium. Information on some foreign populations is presented. The data are discussed in the light of modern evolutionary theory, and a balancing selection model for the maintenance of genetic variability is proposed.