The ras proto-oncogene products are plasma membrane-bound, guanine nucleotide-binding proteins implicated in signal transduction across the plasma membrane. But the signal(s) that activates the ras pathway(s) is not known. In the budding yeast Saccharomyces cerevisiae, the CDC25 gene product acts upstream of Ras proteins, but it has not been clear whether CDC25 function is unique to the S. cerevisiae ras pathway. Here we report that the ste6 gene of fission yeast Schizosaccharomyces pombe is a homologue of CDC25: the ste6 gene product and the CDC25 gene product have significant amino-acid similarity in their C-terminal regions. Like the S. pombe ras1 gene, ste6 is essential for mating. Epistatic interactions indicate that the ste6 gene functions upstream of ras1. We propose that ste6 and CDC25 activate Ras protein through a common mechanism, perhaps by promoting GDP-GTP exchange, even though it seems that the function of Ras protein in budding yeast differs from that in fission yeast. Homologues of ste6 and CDC25 could regulate ras activity in other eukaryotic cells.