The changes in F-actin organization during the cellularization of the Drosophila embryo have been studied with a confocal laser scanning microscope using fluorescein-phalloidin as a specific stain. Particular study has been made of the changes in the organization of the F-actin network associated with the leading edges of the growing membranes. The role of this actin network in the cellularization process is considered. Other actin-containing structures have also been examined, including the cortical actin layer and a conspicuous region of F-actin aggregates, present beneath the level of the forming cell membranes.