The gpal gene of Schizosaccharomyces pombe, which encodes a protein homologous with the alpha subunits of mammalian guanine nucleotide-binding proteins (G proteins), was isolated by cross-hybridization using rat Gi1 alpha and Gx alpha cDNA. The deduced amino acid sequence was about 37% identical with rat Gi1 alpha and Gx alpha proteins and contained three conserved motifs commonly found in all GTP-binding proteins. Disruption of gpa1 was not lethal but conferred sterility and sporulation deficiency on Sch. pombe cells. Thus, the gene is essential for the sexual development and is probably coupled to mating-factor receptors. In contrast to Saccharomyces cerevisiae GPA1, which plays a negative role in mating-factor signal transduction, Sch. pombe gpa1+ apparently has a positive function. A gpa1 transcript of 2.2 kilobases was detected in vegetatively growing cells. A 1.6-kilobase gpa1 transcript appeared in addition to the 2.2-kilobase transcript when cells were derepressed for mating or meiosis.