The effects on beef traits of the recently discovered mutation in the myostatin gene of Blonde d’Aquitaine cattle were estimated in a population segregating this mutation. Three F1 Blonde d’Aquitaine-Holstein crossbred sires were mated either to F1 or Holstein cows to obtain F2 or back-cross calves. Calves were fattened for veal production and slaughtered at 22 weeks of age. Live, slaughter traits and muscle characteristics were recorded. Genotypes at the new polymorphism were determined by RFLP: 19 T/T; 30 T/G and 8 G/G among the 56 calves with genotypes and phenotypes (43 F2 and 13 back-cross). The mutation was shown to have pleiotropic on most of the recorded traits except growth. Muscling and carcass yield were significantly affected positively by the mutation, while fatness and skeletal development were negatively affected. The mutation effects were partially recessive. The mutation also influenced muscle characteristics with higher proportion of fast white glycolytic muscle fibers. All these effects are coherent with published results of former known mutations in myostatin gene and can explain the superiority of Blonde d’Aquitaine muscling.