Meiosis and spore formation in the yeast Saccharomyces cerevisiae are associated with increased expression of sporulation-specific genes. One of these genes, IME2, encodes a putative protein kinase that is a positive regulator of other sporulation-specific genes. We have isolated mutations that cause reduced expression of an ime2-lacZ fusion gene. We found mutations in IME1, a known positive regultor of IME2, and MCK1, a known positive regulator of IME1. We also isolated recessive mutations in 12 other genes, which we designate RIM (Regulator of IME2) genes. Our analysis indicates that the defects in rim1, rim8, rim9 and rim13 mutants are a consequence of diminished IME1 expression and can be suppressed by expression of IME1 from the heterologous ACT1 promoter. These rim mutations also reduced expression of an ime1-HIS3 fusion, in which the HIS3 gene is expressed from the IME1 promoter, and caused reduced levels of IME1 RNA. Although the rim1, rim8, rim9 and rim13 mutant phenotypes are similar to those of mck1 mutants, we found that the defects in ime2-lacZ expression and sporulation of the mck1 rim double mutants were more severe than either single mutant. In contrast, the defects of the rim rim double mutants were similar to either single mutant. The rim1, rim8, rim9 and rim13 mutants also display slow growth at 17 degrees and share a smooth colony morphology that is not evident in mck1 mutants or isogenic wild-type strains. We suggest that RIM1, RIM8, RIM9 and RIM13 encode functionally related products that act in parallel to MCK1 to stimulate IME1 expression.