Mesidotea entomon inhabiting Pauline Cove, YT, is larger in total body length at sexual maturity than M. entomon inhabiting Dolomite Lake, NWT. The sex ratio, diet, habitat distributions, size frequency distributions, and fecundity in the two populations are compared. Total carbohydrate, protein, lipid, chitin, and ash contents of both isopods' life stages are compared. Isopods in the cove show reduced carbohydrate and elevated lipid levels. In the 1975, 1978, and 1979 summers, isopods in the cove were internally infected with an extracellular protozoan parasite; those in the lake were uninfected. The infection is systemic. The prevalence of the protozoan infection increases with increasing host size. The host response involved phagocytosis and encapsulation. Transmission may be transovarially and/or by ingestion of the infective stage. The protozoan also infected Mesidotea sibirica. The infection is related to growth, abundance, habitat, reproduction, and biochemical variables.

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