Immunofluorescence labeling, using a monoclonal antibody developed against actin, revealed the relative abundance and rearrangements of F-actin arrays which occur in cells of the maize root apex as they make the developmental transition from proliferative growth in the meristem to a non-proliferative state in more mature root parts, and during the concomitant process of tissue differentiation. Cells in both the root cap and the quiescent center are depleted of F-actin, whereas it is abundant in cells of the central cylinder but less so in the cortex. The cortical cytoplasm associated with the endwalls of both mitotic and postomitotic cells is characterized by a more intense reactivity to the actin antibody than the longitudinal side walls. A major change in F-actin arrangement occurs in the transitional growth region interpolated between the meristem and the zone of rapid cell elongation. The location and nature of these F-actin rearrangements within the root suggest that the F-actin system might be involved in generating a force associated with the developmental transition of cells from their slow near-isotropic mode of growth close to the base of the meristem, to rapid anisotropic growth which is characteristic of the zone of cell elongation. This attraction notion was strongly supported using specific inhibitors of F-actin and myosin.