A current concept of population biology holds that much genetic variability is maintained in populations as an adaptation tc environmental heterogeneity in time and space (Battaglia, 1963; Bretsky and Lorenz, 1971; Burns and Johnson, 1971; Powell, 1971). Environmental heterogeneity is thought to promote genetic variability by mechanisms of diversifying selection (reviewed in Dobzhansky, 1970). High levels of heterozygosity have also been proposed to increase the developmental homeostasis of individuals confronted by environmental variability, and to confer evolutionary flexibility on populations subjected to long-term changes (Lerner, 1954; Lewontin, 1958). In predictable stable environments high adaptive values might be achieved by homozygous gene ensembles and genetic variability would decline. In recent years the techniques of electrophoresis genetics have yielded estimates usually in the range of 20 to 60% polymorphic loci for outbreeding terrestrial and aquatic animals (tables in O'Brien and Maclntyre, 1969; Kojima et al., 1970; Selander et al., 1970; Gottlieb, 1971). These studies have dealt with species occupying relatively heterogeneous environments and have not tested the hypothesis that genetic variability is a function of environmental variability. We have tested this hypothesis in relation to the deep sea by obtaining estimates of genetic variability in eight species of benthic invertebrates from 1,000-2,000 m in the Atlantic and Pacific Oceans.