Four homogeneous groups of young Charolais bulls have been fattened between 360 and 700 kg with contrasted diets based either on wrapped hay of semi mountain permanent grassland (GW) or corn silage (CS) distributed at two levels of intake (H or L).To be slaughtered at a common carcass weight of 420 kg, the GWH animals needed (on average) 33 more days of fattening than the CSH and the GWL group 38 days more than the CSL group. The average daily gain reached respectively 1660 g (CSH), 1570 g (GWH), 1600 g (CSL) and 1400 g (GWL). Whatever the intake level, the intake of GW was lower (vs CS) at the beginning of the test (-1kg DM/dvsCS) and higher at the end (+1.5 kg DM/d). Adipose tissues of the carcass (67.4 kg) and the 5th quarter (23.8 kg) of CSH were higher than the depots of other groups (58 and 17 kg, respectively). Differences in growth rate had more impact on the dynamics of depots for CS than for GW. The use of properly complemented haylage (DM=60%) allows fattening bulls. But at iso-energy intake, this diet seems to be less efficient than a corn silage diet. Its use must be reasoned considering all other fattening conditions