The present study was conducted to investigate whether in trout there is an increased requirement for dietary methyl linolenate as the levels in the diet are increased. Feeding a diet containing 1% linolenate with 4% laurate resulted in a good growth rate, while the same diet containing 1% linolenate with 9% or 14% laurate resulted in reduced in reduced growth. With such elevated lipid levels, morethan 2% linolenate was required for maximum growth. This result indicates that the elevated dietary laurate levels increase the requirement of rainbow trout for linolenate. On the other hand, dietary laurae levels exerted no significant effects on fatty acid composition, while there were marked changes in fatty acid distribution attributable to the presence of increasing levels of linolenate in the diet. The linolenate-free diet produced high percentages of 16:1, 18:1 and 20:3ω9, and these levels were lowered by the addition of 1% linolenate, regardless of the amounts of laruate in the diets. Consequently, the EFA index, 20:3ω9/22:6ω3, showed low values even in the lowgrowth groups receiving diets containing 1% linolenate along with 9% or 14% laurate. This suggess that it is impossible to judge from the EFA index whether or not fish are receiving a sufficient amount of EFA to maximize growth. The requirement of rainbow trout for linolenate and ω3-HUFA can be postulated to be around 20% and 10% of the dietary lipids, respectively.