Rainbow trout, Oncorhynchus mykiss (Walbaum), fry fed with a diet containing high levels of oxidized lipid, i.e. 46/128-54/164 meq kg-1 oil, fared less well than controls following challenge with Flavobacterium psychrophilum, the causative agent of rainbow trout fry syndrome (RTFS). In three experiments, a greater number of fish fed with the diet containing oxidized lipid died after challenge in comparison to the controls. Moreover, the pathogen was recovered as pure culture growth from the gills and kidney of dead fry. The use of diets rich in oxidized lipids led to dystrophic changes in the liver, kidney and muscle. In comparison, histopathological changes were not observed in fish fed control diets. Overall, a link was demonstrated between the presence of elevated levels of oxidized lipids in the diet and the development of RTFS.