The growth response of fingerling rainbow trout to additions of unsaturated fatty acids with the ω3 structural configuration was determined by substituting into a 10% corn oil control diet either 5% salmon oil, 1% salmon oil, 10% soybean oil, or 1% linolenic acid (in a linolenic acid concentrate). The respective average weight gains of fish fed the above diets for 12 weeks were 4.2, 13.9, 7.9, 9.2 and 8.4 g. Mortalities dropped from 25% in the controls to 6% or less in the other 4 groups. Determination of the fatty acid composition of phospholipids from fish fed the test diets showed that fish given the corn oil control diet had much lower levels of 22:6ω3 (one-half or less) and higher levels of 20:4ω6 and 22:5ω6 than the fish fed the supplemented diets. The neutral lipid fraction did not show any marked differences between experimental groups but did reflect the high levels of linoleic acid present in all the diets.