Bovine leukaemia virus (BLV) is an oncogenic member of the genus Deltaretrovirus of the familyRetroviridae. Recent studies revealed that BLVstrains can be classified into six different genotypes and raised the possibility that another genotype may exist. In order to gain insight into the degree of genetic variability of BLVstrains circulating in the South American region, a phylogenetic analysis was performed using gp51 env gene sequences. The results of these studies revealed the presence of seven BLVgenotypes in this geographic region and the suitability of partial gp51 env gene sequences for phylogenetic inference. A significant number of amino acid substitutions found in BLVstrains isolated in South America map to the second neutralization domain of gp51. A 3D molecular model of BLV gp51 revealed that these substitutions are located on the surface of the molecule. This may provide a selective advantage to overcome immune host neutralization(AU)