Three isoforms of the cytoskeletal-associated, mechanochemical enzymes known as myosin-I have been purified from rat liver; each coisolates with calmodulin. Incubation of the purified myosin-I's with protein kinase C gamma and 32P-ATP results in phosphorylation of the myosin-I heavy chains. After phosphorylation, the myosin-I isoforms bind less radiolabeled calmodulin in binding assays than observed for control samples. Since the purified isoforms are phosphoproteins as determined by immunoblotting with monoclonal antibodies which recognize phosphoamino acids, these results indicate that phosphorylation might play a role in regulation of myosin-I.