Myosin null mutants of Dictyostelium are defective for cytokinesis, multicellular development, and capping of surface proteins. We have used these cells as transformation recipients for an altered myosin heavy chain gene that encodes a protein bearing a carboxy-terminal 34-kD truncation. This truncation eliminates threonine phosphorylation sites previously shown to control filament assembly in vitro. Despite restoration of growth in suspension, development, and ability to cap cell surface proteins, these delta C34-truncated myosin transformants display severe cytoskeletal abnormalities, including excessive localization of the truncated myosin to the cortical cytoskeleton, impaired cell shaped dynamics, and a temporal defect in myosin dissociation from beneath capped surface proteins. These data demonstrate that the carboxy-terminal domain of myosin plays a critical role in regulating the disassembly of the protein from contractile structures in vivo.