Cell-wall-free endosperm cells of the African blood lily (Haemanthus katherinae Baker) were observed while undergoing mitosis on control medium or on medium containing the herbicide trifluralin (α,α,α-trifluoro-2,6-dinitro-N,N-dipropyl-p-toluidine). The percentage of cells proceeding from an initially observed stage to the next stage, in controls versus treatments, was as follows: prophase to prometaphase, 85 versus 16; prometaphase to metaphase, 80 versus 33; metaphase to anaphase, 95 versus 50; and anaphase to cell plate appearance, 98 to 49%.An analysis of time-lapse films of trifluralin-treated mitotic cells reveals that many of the previously reported abnormalities do not represent end points of herbicide action, but rather stages leading to fusion of daughter nuclei. Ultrastructural studies show a decreased number of microtubules and an accumulation of large vesicles in the cell plate region. These vesicles exclude microtubules and cell plate material. It is postulated that the decrease in number of microtubules and in the amount of cell plate material permits movement of daughter nuclei to a central position where fusion occurs.