In the yeast Saccharomyces cerevisiae there are two mating types, a and alpha, which may mate to produce an a/alpha diploid. Mating type is determined by the allele (MATa or MAT alpha) occupying the MAT locus. In a diploid, expression of the MATa1 and MAT alpha 2 genes determines the a/alpha state by regulating the expression of unlinked genes. Previous S1 endonuclease mapping implied that the MATa1 transcript is not processed. We have performed further S1 mapping of this transcript, demonstrating that the MATa1 gene contains two introns, unlike any other characterized nuclear gene in yeast. Both introns contain 5' splice sites and 5'- TACTAACA -3' consensus sequences at the positions predicted by the S1 mapping data. In the splicing-defective rna2 mutant, the mature message disappears rapidly and the precursor RNA accumulates. The RNA processing removes the UGA stop codon which was previously believed to be read-through.