We have cloned a ras gene homologue from fission yeast Schizosaccharomyces pombe and determined its nucleotide sequence. A putative coding sequence for 219 amino acids was found. The sequence contained one set of splicing signals: GTAAGT for a donor sequence, ACTAA for a unique sequence found in introns of yeast genes and TAG for an acceptor sequence, indicating the existence of an intron. The amino-terminal one third of the predicted S. pombe ras protein was nearly perfectly homologous and the next one third moderately homologous to those of mammalian ras proteins. The carboxy-terminal one third showed no homology but terminated with a short conserved sequence Cys-X-X-Z (X being a hydrophobic amino acid) as in other ras proteins. The result of Southern analysis of S. pombe DNA under nonstringent hybridization conditions using our clone as a probe indicated that no other closely related gene may be present in the S. pombe genome. The transcript of this gene could be detected by Northern analysis.