The screening of a flounder cDNA library with a partial sequence of ras gene from flounder (exons 1 and 2) allowed the isolation of two complete cDNA sequences (ras1 and ras2) highly homologous to human Ki-rasb genes. ras1 and ras2 sequences have an homology of 77.3% indicating that they represent two distinct genes, which differ particularly in their 3 regions. ras1 and ras2 intron 1 sequencing revealed an homology of only 50%, confirming that they represent two different genes. Both genes encode for a 188 amino-acid protein, a size characteristic of Ki-rasb proteins. ras1 protein has the stronger homology to the human Ki-rasb protein (99% identity) and ras2 presents a 85.5% of homology. Two transcripts of respectively 2 and 2.8 kb were identified by northern blots with either ras1 or 2 probes. Preneoplastic and neoplastic livers collected from 14 flounder did not present any mutation on the ras2 gene.