We examined the possibility that parasites affect survival rates of anadromous hatchery coho salmon Oncorhynchus kisutch during their period in the wild. Survival was estimated from the rates at which adults returned to the hatchery. The frequency of infection of heart tissue by metacercariae of Nanophyetus sp. was higher in individuals from families with relatively high survival. Various degrees of parasitic and bacterial infection were observed in all groups. We frequently saw extensive infection and tissue reaction to trophozoites of Ceratomyxa sp. (probably C. shasta) in the apparent absence of spores, suggesting that the clinical method now used to determine the presence of Ceratomyxa infection needs to be reassessed.