In Spirogyra, cytokinesis is initiated by a cleavage furrow. When the furrow contacts the elongating interzonal fibers of the telophase spindle, a small phragmoplast appears at the zone of contact, inside of which a cell-plate appears to complete cytokinesis. The use of two sequential mechanisms for achieving cytokinesis may be an intermediate, and possibly a transitional, state in the evolution of the phragmoplast found in higher plants. We tested the relative contribution of the two types of cytokinetic mechanisms by observing the effects of certain drugs on living cells, recorded in time-lapse. The antimicrotubule drug oryzalin causes the spindle to break down rapidly. When applied during early to midcytokinesis, the interzonal spindle collapses onto the cleavage furrow, which continues to grow inward. However, cytokinesis is not complete; examination of such cells 8-12 h later, after the nuclei separate, reveals a small aperture in the center of the cross-wall. This result shows that the phragmoplast/cell-plate is essential for complete cytokinesis, which apparently cannot be accomplished by cleavage alone, and that inward growth of the cleavage furrow is not dependent on microtubules. The antiactin drug cytochalasin D stops cleavage quite rapidly, and the eventual consequences of treatment depend on the stage reached in cleavage. If the cytochalasin is applied early in cytokinesis, cell division is unable to proceed further. Between telophase nuclei the cell does generate a large mass of cytoplasm that contains typical proliferating phragmoplast fibers; cell-plate formation is initiated in this mass but is never able to proceed further, and no coalescence of material into a cross-wall has been recorded. If cytochalasin is applied later, after the cleavage furrow has contacted the interzonal spindle fibers, cleavage stops but now cell-plate formation proceeds and cytokinesis is completed, albeit slowly. This result demonstrates that an interaction between the cleavage furrow and the forming phragmoplast is necessary for the latter to operate normally. These results support our suggestion that Spirogyra illustrates an intermediate stage in the evolution of the phragmoplast, since both cleavage and cell-plate formation are required for cytokinesis. (This suggestion does not require Spirogyra to be on the line of evolution leading to higher plants; rather, Spirogyra appears to be undergoing an evolutionary process similar to what might have occurred in their progenitors.) Furthermore, interaction of the cleavage furrow with the forming cell-plate/phragmoplast is required for cross-wall completion. These results may have significance in considering mechanisms by which higher plants locate their cellplates properly, an ability associated with the evolution and possible function(s) of the preprophase band of microtubules.