Forty-eight heifers, Limousin (LI) and Charolais (CH) breed, were used to evaluate the effect of a basal mixed diet with linseed supplementation (108g/kg DM) on performance and fatty acid (FA) composition of M. longissimus thoracis (MLT) and subcutaneous fat. The feed intake and weight gains were higher, and feed per gain ratio lower in CH heifers than in LI heifers. The muscle lipids and subcutaneous fat of LI heifers contained significantly more MUFA, PUFA, CLA, and less SFA than the lipids of CH heifers. The feeding of extruded linseed significantly increased the concentration of linolenic acid (C18:3n-3), CLA, and decreased PUFA n-6 to PUFA n-3 ratio in lipids extracted from both tissues. Linseed supplementation decreased SFA and increased PUFA in subcutaneous fat. In MLT lipids of linseed-fed heifers PUFA of the n-3 series were increased. It can be concluded that the growth performance of LI heifers was lower, but that their lipids contained more FA that are thought to be important for human health. The feeding of linseed at 108g/kg DM enhanced the nutritive value of beef in terms of FA profile. Breed differences were generally more important than effects of the diet. Dietary effects, however, were more pronounced in PUFA n-3 and arachidonic acid (C20:4n-6).