Developing unicellular fruiting bodies of the protostelid Planoprotostelium aurantium examined by electron microscopy are shown to have a cortical zone of microfilaments that surrounds the lower portion of the sporogenic cell and extends into a cytoplasmic plug that fills the lumen of the tubular, microfibrillar stalk. Labeling with the S-1 fragment of myosin indicates that the microfilaments are actin. A model is proposed in which the actin is involved both in a contractile process that serves to raise the sporogen off the substrate at the tip of the stalk and in a cytoskeletal role by either directly or indirectly localizing the synthesis and orientation of the stalk tube microfibrils. The process of culmination in protostelids is compared to that of the multicellular dictyostelid cellular slime molds.