We have isolated mutants defective in DNA topoisomerases and an endonuclease from the fission yeast Schizosaccharomyces pombe by screening individual extracts of mutagenized cells. Two type I topoisomerase mutants (top1) and three endonuclease mutants (end1) were all viable. The double mutant top1 end1 was also viable and, in its extract, Mg2+- and ATP- dependent type II activity could be detected. Three temperature-sensitive (ts-) mutants having heat-sensitive (hs-) type II enzymes were isolated, and the ts- marker cosegregated with the hs- type II activity. All the ts- mutations fell in one gene (top2) tightly linked to leul in chromosome II. The nuclear division of single top2 mutants was blocked at the restrictive temperature, but the formation of a septum was not inhibited so that the nucleus was cut across with the cell plate. In contrast, the double top1 top2 mutants were rapidly arrested at various stages of the cell cycle, showing a strikingly altered nuclear chromatin region. The type II topoisomerase may have an essential role in the compaction and/or segregation of chromosomes during the nuclear division but also complement the defect of the type I enzyme whose major function is the maintenance of chromatin organization throughout the cell cycle.