The plasmodial stage of Physarum polycephalum contains up to 10(9) nuclei which undergo a naturally synchronous mitosis every 8 h. Nuclear processes such as DNA and RNA synthesis as well as many cytoplasmic processes such as histone synthesis are also synchronous. Physarum polycephalum is therefore widely used in studies of cell-cycle events. This article describes experiments that may help to explain two fundamental biological processes: (1) the mechanism that triggers mitosis, (2) the structural basis of mitotic movement.