Changes in the distribution of myosin during the formation of the cellular blastoderm of Drosophila melanogaster were followed by staining sections of embryos with antibodies to myosin. These were visualized with indirect immunofluorescence. Prior to the start of cell membrane extension myosin is distributed between the nuclear caps as a thin sub-plasmalemma layer. There is also myosin present beneath the surface of the caps. When plasmalemma growth occurs, myosin is associated with the furrow canals, the tips of the advancing membranes. The fluorescence is distributed in an approximately hexagonal pattern around the growth points of each cell. The hexagons are joined up forming a network. It is suggested that this myosin is associated with bundles of microfilaments, orientated parallel to the surface, to form many interlocking contractile rings. The simultaneous contraction of these rings causes the cleavage of the blastoderm. During the first phase of membrane growth, myosin is also associated with the apical surfaces of the forming cells. At this stage these surfaces are rich in microvilli. However, by the time the furrow canals have reached the bases of the cells much of this myosin has disappeared. At about this time the apical surface becomes taut with a loss of the microvilli.