The mes1+ gene of the fission yeast Schizosaccharomyces pombe is essential for the second meiotic division. We have cloned a 1.1-kb HindIII fragment containing mes1+ by complementation from an S. pombe genomic library. Sequencing of the genomic and cDNA fragments indicates the existence of one small intron of 75 nucleotides, although both the 5'(G/GTTAGT) and 3'(CAG/T) intron-exon junctions deviate from the consensus sequences proposed for S. pombe. The putative translation product of the mature mes1+ mRNA is a 11-kDa protein of 101 amino acids which has no significant homology to any previously-reported proteins. Disruption of mes1 has no effect on cell growth but causes an arrest of meiosis before the second meiotic division. Northern-blot analysis revealed that mes1+ was preferentially transcribed under conditions of nitrogen starvation. When a h90 homothallic strain was shifted to a nitrogen-deficient medium, a pre-mRNA accumulated and then was gradually processed to generate a mature mRNA. This splicing did not occur in either a heterothallic haploid strain or in a homothallic mei2 mutant strain which was defective in the initiation of meiosis. Expression of the first exon alone was not able to suppress the mes1 null allele. These results indicate that mes1+ is required for the completion of meiosis, that splicing is required for the function of the mes1+ gene, and that this splicing requires the function of the mei2+ product.