The prevalence and geographic distribution of Nadelspora canceri, a recently described microsporidian parasite of the Dungeness crab Cancer magister, was studied and prevalences were analyzed by sex and size of host crab and date of collection. Nadelspora canceri was widely distributed along the United States Pacific Coast in estuaries from Bodega Bay, California, to southern Washington. Prevalence of the parasite was high in crabs from most estuaries sampled, ranging from a low of 0.4 % (Gray's Harbor, Washington) to 41.2 (Tillamook Bay, Oregon) with an overall mean of 14 X . The parasite also occurred offshore of California and Oregon, but was not found in Puget Sound, Washington or in Glacier Bay, southeastern Alaska. The prevalence of infection in crabs captured in the open ocean was low (0.3%), and in crabs taken from nearshore at estuary mouths it was intermediate between estuarine and open-ocean levels. The infection was most prevalent (22.4 %) in 2 yr old crabs (13 cm carapace width) and males had approximately 2.5 times hlgher rate of infection than did females (10.7 vs 3.3 % ) No significant annual or seasonal variations in infection prevalence were detected.