We provide the first link between a known molecular motor and morphogenesis, the fundamental process of cell shape changes and movements that characterizes development throughout phylogeny. By reverse genetics, we generate mutations in the Drosophila conventional nonmuscle myosin (myosin II) heavy chain gene and show that this gene is essential. We demonstrate that these mutations are allelic to previously identified, recessive, embryonic-lethal zipper mutations and thereby identify nonmuscle myosin heavy chain as the zipper gene product. Embryos that lack functional myosin display defects in dorsal closure, head involution, and axon patterning. Analysis of cell morphology and myosin localization during dorsal closure in wild-type and homozygous mutant embryos demonstrates a key role for myosin in the maintenance of cell shape and suggests a model for the involvement of myosin in cell sheet movement during development. Our experiments, in conjunction with the observation that cytokinesis also requires myosin, suggest that the processes of cell shape change in morphogenesis and cell division are intimately and mechanistically related.