In an especially good sample series of the deep-sea isopod Eurycope iphthima Wilson, substantial variation occurs in the cephalic rostrum and in several aspects of its population structure: the size-frequency distribution, size of maturation of males and females, proportions of female stages, proportions of juvenile stages, and maximum size of adults. Comparisons of samples from different depths (2,500 to 4,800 m) and different localities (northeastern and central Atlantic Ocean) show the variation to be distinctly depth-related: populations from similar depths are more similar than those from different depths. Because of continuity in other taxonomically important characters, the variation must be clinal and reflects adaptation to local environments, directed genetic variation, or both. The parallel clines in rostral morphology and population structure are most apparent on the continental slopes and rises off Ireland and in the Bay of Biscay. Bimodality in the population structure of the deeper populations is unusual; two hypotheses for its explanation are discussed, although evidence to support either is currently unavailable.