Abstract.Examination of the digestive tract of rainbow trout fry infected withHexamita salmonisrevealed no pathological changes in the epithelial cells of the pyloric caeca or upper intestine even where the parasites were closely apposed to the brush border of these cells. No invasion of the epithelium by the parasites was observed, although examples of shrinkage necrosis (apoptotic bodies) of epithelial cells were often found. The ultrastructure of the parasite resembled that of the closely relatedGiardia lamblia. Despite the lack of pathological changes to the intestinal epithelium, treatment of the fish to remove the parasites greatly reduced mortalities.