Microfilament bundles (MFBs) of F-actin were observed by fluorescence microscopy in cells ofSpirogyra treated with rhodamine-phalloidin. Four types of MFBs could be recognized on the basis of locality and appearance: those dispersed in the cytoplasm near the cell surface; those beneath the plasma membrane running parallel to each other; those at the edges of the chloroplast; and those surrounding the nucleus. Each type exhibited a unique behavior during the cell cycle. Microfilament bundles dispersed in the cytoplasm came together at the middle of the cell to form a fibril ring at the mitotic prophase. The fibril ring decreased in diameter, causing the development of a furrow in the protoplast that progressed from the outside to the inside. After the completion of furrowing, the MFBs in the fibril ring dispersed beneath the plasma membrane. Microfilament bundles surrounding the nucleus formed a net-like cage which became invisible at the mitotic anaphase, while MFBs seen at the chloroplast edges persisted there during the cell cycle without changing their position. Parallel MFBs running perpendicular to the long axis of the cell were seen at all stages in the cell cycle.

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