Temperature-sensitive mutants of the pat1 gene of Schizosaccharomyces pombe are able to produce poorly viable spores from the haploid state without the requirement for nitrogen starvation if exposed to the restrictive temperature. Three lines of evidence strongly suggest that this gene codes for a factor whose physiological role is inhibition of initiation of meiosis. First, pat1 haploids arrested in G(1) phase undergo apparent premeiotic DNA synthesis at the restrictive temperature. Second, the pat1 gene was found to exert its function at a specified stage of the meiotic pathway by comparison with other meiotic mutants. Third, meiotic recombination and chromosome segregation take place quite normally in pat1 diploids at the restrictive temperature, resulting in the production of highly viable spores. A negative control mechanism for meiosis is proposed.