From a Schizosaccharomyces pombe genomic library we have isolated the gene cwl1+ that causes cell lysis when it is overexpressed in the absence of an osmotic stabilizer. Southern hybridization showed that cwl1+ exists as a single copy in the S. pombe genome. The cwl1+ gene nucleotide sequence revealed a putative open reading frame of 924 bp encoding a polypeptide of 308 amino acids with a calculated Mt of 27000. The cwl1+ DNA hybridizes to a major RNA transcript of 1.5 kb whose 5' end maps at a position 452 bp upstream from the predicted translation start. Comparison of the amino acid sequence with those included in the current databases, showed no significant similarity to any known sequences. Cells overexpressing the cwl1+ gene under the control of the S. pombe nmt inducible promoter displayed a reduced cell wall content, were unable to separate after division and lysed drastically in the absence of osmotic stabilizer. Disruption of the cwl1+ gene caused no noticeable phenotype.