Cytokinesis in higher-plant cells involves the formation of a cell plate in the interzone between the separating chromatids. The process is directed by the phragmoplast, an array of microtubules, actin filaments, and membranous elements. To determine if the role of actin in cytokinesis is dependent on myosin, we treated stamen hair cells of Tradescantia virginiana L. with 2,3-butanedione monoxime (BDM), an inhibitor of myosin ATPase and ML-7, a specific inhibitor of myosin light-chain kinase. Treatment with BDM resulted in a tilted cytokinetic apparatus during early initiation and a wavy cell plate with curved phragmoplasts during late lateral expansion. Treatment with ML-7 also resulted in inefficient late lateral expansion of the cell plate, with effects ranging from slower expansion to complete inhibition. Taken together, these results implicate myosin in the control of cell plate expansion and alignment.