Dissection of complex processes using model organisms such as yeasts relies heavily upon the use of conditional mutants. We have generated a collection of fission yeast mutants sensitive to dimethylsulphoxide (DMSO). Among these we have found a mutant in the Schizosaccharomyces pombe orthologue of the Saccharomyces cerevisiae SEC13 gene, which fails to cleave the division septum. Generation of a null allele demonstrates that the S. pombe sec13 gene is essential.