Cytological observations have shown that the presence of unstable minichromosomes can delay progression through the early stages of mitosis in fission yeast (Schizosaccharomyces pombe), suggesting that such minichromosomes may provide a useful tool for examining the system that regulates the coordinated segregation of chromosomes. One such unstable minichromosome is a large circular minichromosome. We previously showed that the mitotic instability of this minichromosome is probably due to the frequent occurrence of catenated forms of DNA after replication. To identify genes involved in the regulation of chromosome behavior in mitosis, we isolated mutants which stabilized this minichromosome. Three loci (sta1, sta2, and sta3) were identified. Two of them were found to be suppressors of temperature-sensitive mutations in cdc2, which encodes the catalytic subunit of muturation promoting factor (MPF). They show no linkage to, and are thus different from, suc1, and cdc13, previously identified as genes that interact with cdc2. The other mutation mapped to a gene previously identified as being required for the correct formation of the mitotic spindle. Data provided in this study suggest that the sta genes are involved in the regulation of spindle dynamics to ensure proper chromosome segregation during mitosis.