Primary mesenchyme formation in sea urchin embryos occurs when a subset of epithelial cells of the blastula move from the epithelial layer into the blastocoel. The role of microfilaments in producing the cell shape changes that characterize this process, referred to as ingression, was investigated in this study. f-Actin was localized by confocal microscopy using labeled phalloidin. The distribution of f-actin was observed before, during, and after ingression and was correlated with cellular movements. Prior to the onset of ingression, staining became intense in the apical region of putative primary mesenchyme and disappeared following the completion of mesenchyme formation. The apical end of these cells constricted coincidentally with the appearance of the intensified staining, indicating that f-actin may be involved in this constriction. In addition, papaverine, a smooth muscle cell relaxant that interferes with microfilament-based contraction, and that was shown in this study to inhibit cytokinesis, diminished apical constriction and delayed ingression. Despite this interference with apical constriction, the basal surface of ingressing cells protruded into the blastocoel. It is suggested that apical constriction, while not necessary for ingression, does contribute to the efficient production of mesenchyme and that protrusion of the basal surface results from changes that occur independent of apical constriction. © 1992 Wiley-Liss, Inc.