The pathological effects of the blood fluke, Sanguinicola klamathensis Wales 1958, was studied in the cutthroat trout host (Salmo clarki). Cutthroat trout fingerlings of a non-infected group and a group experimentally infected by exposure to 6,000 Fluminicola fusca snails, with a 6% prevalence of infection with S. klamathensis, were maintained for 7 months. The experimental group had 80% mortality after 3 months exposure to the blood flukes. Histopathology revealed a progressive infection with fluke eggs and miracidia within the gills, kidney, and heart as evidenced by necrosis and calcification of heart and kidney tissue, and hyperplasia of the gills.