The cps8 mutation which confers supersensitivity to a spindle poison, Isopropyl N-3-chlorophenyl carbamate (CIPC), in the fission yeast Schizosaccharomyces pombe was investigated. The cps8 mutant accumulated enlarged multinucleate cells in the stationary phase under normal growth conditions. The mutant was highly lethal at 36.5 degrees C in a fresh growth medium but not in a saline solution where the cell cycle ceases quickly. Lethality at high temperature was significantly suppressed by cdc1 or nda2 mutation which blocks nuclear division, but not by hydroxyurea treatment or cdc22 mutation which blocks DNA synthesis. A cdc10 cps8 double mutant remained lethal to high temperature, suggesting this double mutant to bypass the requirement for cdc10+ indispensable for the cell cycle start in a wild-type cell. After being transferred to a fresh medium at 36.5 degrees C, the multinucleate cells rapidly divided with aberrant nuclear segregation. Thus, cps8 mutation allows cells to undergo mitosis without DNA replication at the restrictive temperature. The cps8 gene was mapped on the left arm of chromosome II closely linked to but distinct from cdc2 locus.