The Rho family of GTP-binding proteins plays critical roles during myogenesis induction. To elucidate their role later during myogenesis, we have analyzed RhoA function during myoblast fusion into myotubes. We find that RhoA activity is rapidly and transiently increased when cells are shifted into differentiation medium and then is decreased until myoblast fusion. RhoA activity must be down-regulated to allow fusion, because expression of a constitutively active form of RhoA (RhoAV14) inhibits this process. RhoAV14 perturbs the expression and localization of M-cadherin, a member of the Ca²⁺-dependent cell-cell adhesion molecule family that has an essential role in skeletal muscle cell differentiation. This mutant does not affect N-cadherin and other proteins involved in myoblast fusion, β1-integrin and ADAM12. Active RhoA induces the entry of M-cadherin into a degradative pathway and thus decreases its stability in correlation with the monoubiquitination of M-cadherin. Moreover, p120 catenin association with M-cadherin is decreased in RhoAV14-expressing cells, which is partially reverted by the inhibition of the RhoA effector Rho-associated kinase ROCK. ROCK inhibition also restores M-cadherin accumulation at the cell-cell contact sites. We propose that the sustained activation of the RhoA pathway inhibits myoblast fusion through the regulation of p120 activity, which controls cadherin internalization and degradation.