In this study, the basis for the classification of virus isolates grouped within the genus Coltivirus, family Reoviridae, is discussed. Sequences of dsRNA segments from American (segments 9-12), European (segment 12) and Asian (segments 7-12) isolates were characterized and polythetic criteria were defined for their taxonomic classification. These criteria (including sequence analysis) permitted the different species to be distinguished and classified into two groups. In both groups, subgroups were defined according to the degree of homology between the genomic sequences. American and European isolates are classified within group A, which includes subgroups A1 (Colorado tick fever virus species) and A2 (Eyach virus species). Asian isolates are classified in group B, which includes subgroups B1 (JKT-7075 virus species) and B2 (JKT-6423 virus species). The proteins encoded by the sequenced genomic segments were analysed. This allowed the identification of dsRNA binding domains in the proteins encoded by segment 8 of subgroup B1 isolates and segment 12 of subgroup B2 isolates. A conserved pattern of amino acids in segment 7 of group B isolates matched sequences found in the catalytic domains of protein kinases.