The effects of modified atmosphere (60% O2, 30% CO2, 10% N2) were compared with respect to the main microbial parameters and to some fundamental physiochemical characteristics (pH and colour) of beef steaks packaged under vacuum stored at 4°C. In modified atmosphere, total mesophilic and psicotrophic bacterial cont and other bacterial rates (Pseudomonaceae, Brochotrix thermosphacta, Enterobacteroaceae) grew statistically at 14° day. In beef steks processed with vacuum packaging the anaerobic psicotrophic bacterial count grew statistically significantly at 14° day. Beef under modifoed atmosphere showed higher yellowness (b*) and Hue (H*) than beef under vacuum.