Myostatin inhibits myogenesis. Therefore, we sought to determine if mice lacking the myostatin gene [Mstn(-/-)] would lose less muscle mass than wild-type mice during 7 days of hindlimb suspension (HS). Male Mstn(-/-) and wild-type (C57) mice were subjected to HS or served as ground-based controls (n = 6/group). Wild-type mice lost 8% of body mass and ∼13% of wet mass from biceps femoris, quadriceps femoris, and soleus, whereas the mass of extensor digitorum longus (EDL) was unchanged after HS. Unexpectedly, Mstn(-/-) mice lost more body (13%, P < 0.05) and quadriceps femoris (17%, P < 0.05) mass than wild-type mice and lost 33% of EDL mass (P < 0.01) after HS. Protein expression of myostatin in biceps femoris and quadriceps femoris was not altered, whereas expression of MyoD, Myf-5, and myogenin increased in wild-type mice and tended to decrease in muscles of Mstn(-/-) mice. These data suggest that HS induced myogenesis in wild-type mice to counter atrophy, whereas myogenesis was not induced in Mstn(-/-) mice, thereby resulting in a greater loss of muscle mass.