We have cloned the ras homologue from the mangrove rivulus (Rivulus marmoratus) after low-stringency plaque hybridization of the mangrove rivulus genomic DNA library. This mangrove rivulus ras homologue showed a 76% amino acid homology to the human R(related)-ras gene with identical exon/intron boundary and was named the mangrove rivulus R-ras gene. The mangrove rivulus R-ras gene spanned 1.8 kb and consisted of at least 5 exons. The exon/intron boundaries coincided with the rule of GT/AG of consensus splice acceptor and donor sequences. To our knowledge this is the first report that fish also have the R-ras gene with identical exon/intron boundaries and extensive homology with human.