The function of the ras gene of Schizosaccharomyces pombe has been studied by constructing null and activated alleles of this gene. An activated allele (Val (12)) inhibits conjugation but has no effect on cell growth, entry into stationary phase or sporulation. The phenotype of Val (12) is distinct from that caused by elevating the intracellular level of cAMP. This supports the hypothesis that ras of fission yeast does not modulate adenylate cyclase in a manner analogous to S. cerevisiae RAS. Introduction of a human ras sequence into fission yeast cells containing a non-functional null allele of ras restored the sexual differentiation process thus indicating that the human sequence can complement S. pombe ras. Our data suggest that although ras genes are highly conserved across a considerable evolutionary divide, the cellular function of the ras gene product varies in different organisms.