In order to assess the possible role of microfilaments (Mfs) in events preceding plant cell division, actin was localized in root cells of Allium cepa and Tradescantia virginiana by immunofluorescence microscopy. The distribution of Mfs was compared to that of microtubules (Mts) by means of dual localizations employing both antiactin and antitubulin. Cycling interphase cells contain Mfs that extend into all regions of the cytoplasm in random fashion. Prior to the rearrangement of the cortical Mt array into the initial broad preprophase band (PPB), the number of Mfs in the cytoplasm decreases, while a new population appears in the cortex. The cortical Mfs, which usually occupy the entire cell surface, are aligned parallel to the cortical Mts. When the initial PPB appears, these Mfs still cover the cortex or are arranged as a broad band encompassing the PPB. As the PPB narrows, the Mfs are also confined to an increasingly restricted zone usually wider than the PPB.than the PPB. When the PPB reaches its narrowest, densest configuration, aligned Mfs are excluded from the band proper, while others appear in flanking regions of the cortex. From prometaphase through anaphase, cortical Mfs are largely restricted to the ends of the cell overlying the spindle poles; they also tend to become more randomly oriented. Little or no actin is present in the spindle. During telophase, the two zones of aligned cortical Mfs over the ends of the cell gradually disappear and are replaced by new interphase networks. These changes provide additional data on the possible control of PPB organization by actin, and in addition indicate that the cortex may be the origin of the actin that aggregates at the spindle poles during cytochalasin treatment. © 1992 Wiley-Liss, Inc.