The epizootiology and histopathology of the myxosporean Parvicapsula sp. was studied during monthly health surveys of 4 groups of coho salmon Oncorhynchus kisutch at a commercial farm in Puget Sound, Washington, USA, from 1984 to 1986. No Parvicapsula sp. was detected in histological samples taken from juvenile fish in fresh water, but the parasite was detected in fish from all groups 2 to 8 mo after transfer to seawater net pens. Groups placed in seawater net pens in November and December had a higher prevalence of infection, and a longer period of continuous detected infection, than those introduced into net pens in May. For the groups transferred to seawater in November and December, the highest infection prevalence (45 to 90%) was detected during the following March and April. Among 13 tissues examined histologically, only the pseudobranch and kidney were positive for Parvicapsula sp., with 26 (62%) of 42 positive fish showing infections only in the pseudobranch, 5 (12%) showing infections only in the kidney, and 11 (26%) showing infections in both organs. Both the pseudobranch and kidney were apparent primary infection sites, but pseudobranch infections appeared to persist longer in a population. Pseudobranch infections were frequently heavy and associated with extensive inflammation and necrosis of filament and lamellar tissues. The kidney had been the only infection site reported for Parvicapsula sp. in previous studies of coho salmon.