We show that DNA topoisomerase II (top0 II) is continuously required for mitotic chromosome changes in Schizosacchammyces pombe. We constructed coldsensitive (cs) or temperature-sensitive(ts) strains mutated in the genes coding for topo II (fop2) and S-tub& Iin (n&3). The ATP-dependent activity of the t0p2~* gene product is cs in vitro. The cloned f0p2~= gene sequence predicts an amino acid substitution. A cs top2-cs nda3 double mutant at 20% shows long, entangled chromosomes, which condense and separate upon the shift to permissive temperatures. If spindle formation is prevented at permissive temperatures, the chromosomes condense but do not separate. Thus topo II is required for final chromosome condensation; moreover, pulse-shift experiments show that topo II is required for chromatid disjuction. Experiments with ts fop2-cs nda3 cells show that topo II is also required for chromosome separation in anaphase: inactivation of topo II and activation of P-tubulin allow normal spindle formation but result in% treaked” chromosomes.