We investigated the growth of rainbow trout (Salmo gairdneri) fed diets containing an animal fat (lard) as a source of energy. Nine diets, identical in gross energy, but containing three levels of protein, were each fed to duplicate groups offish for 14 weeks. The control diets contained 22% herring oil; in the test diets, 33 and 50% of the dietary herring oil was replaced by lard. The final fish weight, feed consumption, feed efficiency, and fish liver size were about the same in the groups offish receiving control and test diets containing the same protein level.