This comparative study on the fecundity and age structure of the isopodMesidotea entomon(L.) from the Canadian western Arctic is one of the few studies on this species in it's two distinct habitats. In the marine population, year class segregations varied in density both in time and space. In contrast, the freshwater population remained relatively stable. Trimodal/bimodal size groupings suggests that this species takes 2–3 years to reach sexual maturity. Sexual size dimorphism in which the male is the larger sex was evident in both populations. The marine isopods sexually matured at a larger body size than those in freshwater and females produced larger and a greater number of eggs per unit length than those in freshwater. Fecundity was positively correlated with size.