-In two large karst areas, one in West Virginia and the other in Virginia, populations of Gammarus minus (Amphipoda) in caves have significantly larger body size, longer antennae and significantly reduced eye size compared with populations occurring in springs. We examined genetic structure (allozyme variation) and morphological differentiation (ommatidia number) of caveand spring-dwelling populations of G. minus in the Ward's Cove karst drainage in Virginia. We found that: (1) there is significant genetic and morphological differentiation between the hydrologically connected cave and spring populations, but (2) there is little genetic or morphologic differentiation between the two hydrologically connected cave populations. These patterns are essentially identical, both qualitatively and quantitatively, to those reported for the geographically disjunct West Virginia populations. These data support the hypothesis that the similar morphologies of cavedwelling G. minus in West Virginia and Virginia are driven by similar selection pressures. These results also suggest that the taxonomic status of cave and spring populations, currently considered to be conspecific, should be re-evaluated.