Forty male Ile-de-France lambs (10 blocks of 4 homologous lambs) were used to study the effects of four feeding systems on muscle fatty acids (FA): raising and finishing on cool-season grasses (G), raising on the same grasses and stall-finishing, indoors, on concentrates and hay, respectively, for 22 (GSS) or 41 days (GSL), and stall-feeding indoors on concentrates and hay during both growing and finishing periods (S). Twenty-four lambs only (6 blocks) were retained for comparison of growth performances, lipid content in the longissimus thoracis muscle (LT) and their FA composition according to treatment. The 16 other lambs (4 blocks) were removed from the comparison, due to a large spread in the growth of the lambs towards the end of the trial. No significant effects of treatment were seen on the rate of growth (221, 228, 243 and 245 ± SE 8.0 g/d, respectively, for G, GSS, GSL and S groups), and the lipid contents of the LT (2.22, 2.16, 2.17 and 2.52 ± SE 0.11 g/100 g fresh tissue). Grazing, lowered n − 6 PUFA (polyunsaturated fatty acids), and increased n − 3 PUFA and C18:2 c9t11 (conjugated linoleic acid cis9, trans11) compared to concentrate feeding. The main effects of grazing were not removed by a short period of finish indoors on concentrate (GSS group), but C20:4 n − 6 and C22:6 n − 3 contents achieved the lowest contents in this group, with significant differences from the values observed for GSL and S groups (C20:4 n − 6) or from the three other groups (C22:6 n − 3). After a longer period of finish on concentrate (GSL group), C18:3 n − 3 (linolenic acid), C18:2 c9t11 and long chain (LC) n − 3 PUFA were brought to the levels observed in the S group. In terms of adequacy for human health, the C18:2 n − 6/C18:3 n − 3 ratios were favourably low in the four groups (2.6, 3.6, 4.9 and 5.2 ± SE 0.7, respectively, for G, GSS, GSL and S groups), the level observed in the case of G group being significantly lower than for the three other groups and the level observed for GSS group being significantly lower than for the GSL and S groups.