The complete gene of RTV DNA methyltransferase ( MTase) was amplif ied, cloned and sequenced. The ORF consists of 642 bp which codes for a protein of 214 aa with a predicted molecular mass of 24. 8kD. Sequence analysis of MTase gene shows much higher identity to the species of genus Ranavirus ( 96% - 97%) than to lymphocystis disease virus( 56% ) , the type species of the genus Lymphocystivirus, indicating again that RTV was the member of the genus Ranavirus; just as the other vertebrate iridoviruses, the MTase gene of RTV contains the first four highly conserved motif s of cytosine MTases but the fifth motif, reponsible for DNA binding specif icity, is missing; Among the virus of RTV, doctorfish iridovirus and largemouth bass ranavirus, the very different identity between MTase and MCP gene suggests that the gene used as target to estimate the evolution of the iridoviruses should be suitable.