The actin-activated myosin ATPase activity of myosin isolated from proliferative myoblasts is dependent on the phosphorylation of the myoblast myosin. The kinase responsible for this phosphorylation is not dependent on Ca 2 ions for its activity. Preliminary experiments suggest that following cell fusion the kinase initially disappears. It may re-emerge following sarcomere formation in a Ca 2 ions dependent form.