In the budding yeast Saccharomyces cerevisiae, the preservation of the mating competent haploid (a or α) and the mating incompetent diploid (a/α) is necessary to prevent aneuploidy. Once haploid cells respond to pheromone, the mating-specific signal transduction pathway is activated, and the MAP kinase Fus3 phosphorylates two specific repressor proteins Rst1 and Rst2 (also known as Dig1 and Dig2) to promote Ste12-dependent transcription of mating-specific genes. In contrast, diploid cells cannot mate because genes that encode components of the mating pathway are repressed through the combined action of the Mata1–Matα2 and Matα2–Mcm1 repressors. Surprisingly, repression of Ste12 by Rst1 and Rst2 is essential for diploid sterility. Homozygous deletion of both RST1 and RST2 (rst–) causes a/α diploid cells constitutively to express a-specific genes and mate preferentially as a-cells. This phenotype is sensitive to Ste12 dosage, as removal of one copy of STE12 completely reduces the ectopic activation of a-specific genes. The Matα2–Mcm1 complex, which normally represses a-specific genes, is defective in rst– diploids because Matα2 is destabilized in rst– diploids, possibly as a consequence of its relocalization from the nucleus to the cytoplasm. This study finds that Rst1 and Rst2 are necessary for the a/α diploid cell type. Rst1 and Rst2 are required in order to prevent the amplification of a robust Ste12 transcriptional programme that appears to over-ride Matα2-dependent repression of haploid and a-specific genes.