Data on the dynamics of the nuclear cycle in the coenocytic fungus, Saprolegnia ferax, were obtained by a combination of serial-section electron microscopy, DNA measurements and nuclear counts of vegetative hyphae and germinating spores. These methods are the only ones capable of giving the timing of the stages of division at the level of resolution of thin-section electron microscopy. All nuclei observed contained various arrays of kinetochore microtubules and were thus defined as being in mitosis. Among this population the mean percentage (corrected for age gradient) of each stage was as follows: prophase with one pair of centrioles, 28.5; prophase with two pairs of centrioles, 41.9; metaphase, 23.8, anaphase, 0.8 and telophase, 4.4. The total duration of the nuclear cycle was calculated as a mean of 76 min and a minimum of 30 min from which the duration of each phase was calculated. Based on these figures and observed spindle lengths, rates of spindle elongation up to 31 micron/min, chromosome-to-pole movements up to 8.5 micron/min and microtubule polymerization up to 30.8 micron/min were deduced. Because transitions between the pre-metaphase stages were rare, it is concluded that mitosis progresses initially as a series of rapid steps with intervening morphologically quiescent stages.