Video-connected fluorescence microscopy was introduced to study the yeast nuclear chromatin region. It was defined as the nuclear area where a DNA-binding fluorescent probe 4',6-diamidino-2-phenylindole specifically bound and fluoresced. The 3-dimensional feature of the mitotic chromatin region was deduced by analysing the successive video images of a cell viewed at different angles. By investigating synchronous culture of the wild-type fission yeast Schizosaccharomyces pombe, we found sequential structural alterations in the chromatin region during mitosis. The steps found include the compaction of the chromatin region from the regular hemispherical form, the formation of a U-shaped intermediate and the rapid segregation into 2 daughter hemispherical forms. Six cs cdc mutants, apparently blocked in mitosis, were observed by fluorescence microscopy. Under the restrictive conditions their chromatin regions exhibited either hemispherical, compact, disk-like, U-shaped or partially segregated chromatin regions. Two mutants showed anomalous nuclear locations. The results of the temperature shift-up experiments of the highly reversible KM52 and KM108 strains supported the above scheme of sequential alterations in the chromatin region.