GPA1 protein of Saccharomyces cerevisiae is homologous to the alpha subunit of mammalian G protein. GPA1 transcript was found in haploid cells but was not detected in diploid cells. Disruption of GPA1 resulted in a haploid-specific lethal phenotype, indicating that GPA1 is a haploid-specific essential gene for cell growth. Upon regulation of expression of GPA1 by the galactose-inducible GAL1 promoter, the loss of GPA1 function was found to lead to cell-cycle arrest at the late G1 phase. Mutants that suppress the lethality of the gpa1::HIS3 mutation showed a sterile phenotype that was not cell-type-specific. These results suggest that GPA1 protein may control the signal for mating-factor-mediated cell-cycle arrest.