The phosphorylation pattern of numerous proteins in the soluble extracts of chick embryonic muscle cells changes dramatically during myogenesis. One of these proteins, the 63 kDa protein, whose phosphorylation state declines during the differentiation process, was identified as the myristoylated alanine-rich C kinase substrate (MARCKS), a major, specific substrate of protein kinase C (PKC). This decrease in the phosphorylation state of MARCKS was due to a decrease in the level of protein in the cytosol with a simultaneous increase in its level in the membrane fraction. Immunostaining of the cultured myoblasts also revealed that MARCKS translocated from the cytosol to the plasma membrane and to the peripheral region of nuclei as the mononucleated myoblasts fused to form multinucleated myotubes. Immunoprecipitation with an anti-PKC-theta antibody, but not with the antibodies against the other PKC isoforms, such as conventional PKC-alpha, novel PKC-delta, and novel PKC-epsilon, inhibited phosphorylation of MARCKS. Moreover, expression of PKC-theta was found to be down-regulated during the course of myogenic differentiation. In addition, treatment of the cells with PMA, which activates PKC-theta and hence increases the phosphorylation state of MARCKS, reversibly inhibited both MARCKS translocation and myoblast fusion. These results suggest that MARCKS is preferentially phosphorylated by PKC-theta in cultured myoblasts and that the down-regulation of PKC-theta; is partly responsible for MARCKS translocation during myogenesis. These results also suggest that PKC-theta-controlled MARCKS translocation is associated with, or a requisite event for, myoblast fusion.