Sporulation-deficient mutants were isolated from a homothallic strain of Saccharomyces cerevisiae. Sporulation was induced in these mutants by procedures to sporulate the products of protoplast fusion between mutants and wild-type strains. Spores formed in this way were crossed to wild-type strains in order to analyze them genetically. Twenty-three genes essential to sporulation were identified by tetrad analysis and complementation tests. Gene symbols spoT1 to spoT23 were tentatively assigned to them. These mutants fell into four classes by examination of premeiotic DNA synthesis and meiotic nuclear division: (i) Premeiotic DNA synthesis did not occur (spoT1 - spoT11); (ii) premeiotic DNA synthesis occurred but meiosis I did not occur (spoT12 - spoT15); (iii) meiosis II did not occur (spoT16 - spoT18); (iv) meiosis II occurred but mature spores were not formed (spoT19 - spoT23). Genes spoT4, spoT8, spoT20, and spoT23 were mapped on chromosomes IV, II, XVI and XI, respectively. SpoT18-1 was a UAG nonsense mutation.