The microsporidian parasite Nadelspora canceri infects Dungeness crabs, Cancer magister, along the United States Pacific Northwest coast. The prevalence and seasonal variation of N. canceri in Dungeness crabs from Alsea Bay, Oregon, are described based on examination of 2991 crabs collected at monthly intervals from October, 1991 to June, 1993. The average prevalence in monthly samples was 21.0% and ranged from 8.2% to 33.0%. No significant differences in monthly or seasonal parasite prevalence were observed. A total of 3061 Dungeness crabs was examined from an additional seven Pacific Northwest estuaries and Puget Sound to document the geographic distribution of N. canceri and the prevalence of the parasite in these locations. The estuaries sampled and the prevalences observed were: Humboldt Bay, California (14.6%), Coos Bay (10.6%), Yaquina Bay (2.0%), Tillamook Bay (41.2%), and Nehalem Bay, Oregon (14.2%), Willapa Bay (6.9%), and Grays Harbor Washington (0.44%). Dungeness crabs were examined from the Dungeness spit, Kala Point, and Mukilteo areas in Puget Sound and no infected crabs were found. A total of 9317 male Dungeness crabs > 15.9 cm carapace width (CW) captured in the commercial ocean crab fishery was examined for N. canceri and 27 (0.3%) were infected with the parasite. No infections were found in crabs smaller than 3.0 cm CW and the prevalence of infection generally increased with crab size reaching a peak of 22.2% in 14 cm CW crabs. The overall infection prevalence in male crabs (19.2%) was more than twice that of female crabs (8.0%), and of the 821 infected crabs found, 629 (76.6%) were males. The mortality of laboratory-held Dungeness crabs naturally infected with N. canceri was compared to that of uninfected crabs in two separate experiments and in both cases a significantly higher mortality was observed for infected crabs. Nadelspora canceri infections were established in both juvenile and adult Dungeness crabs that were fed parasite spores in laboratory experiments indicating that transmission is direct and intermediate hosts or vectors are not required for transmitting the parasite between hosts. / Graduation date: 1995