cDNA cloning of the double-stranded RNA genome of Chuzan virus, a member of the Palyam serogroup orbiviruses, was carried out and the complete nucleotide sequences of RNA segments 2, 3, 6 and 7, encoding the major capsid proteins VP2, VP3, VP5 and VP7, respectively, were determined. The individual segments had single open reading frames and short inverted repeats adjacent to the conserved terminal sequences. Comparative sequence analysis with other serogroups of the genus Orbivirus suggested that VP2 is the principal determinant of serotype specificity and the neutralizing antigen of the Palyam serogroup. VP5 is also considered to be associated with antigenic variability. Both VP3 and VP7 probably contain serogroup-specific epitopes. Phylogenetic profiles demonstrated that the Palyam serogroup virus is more closely related to African horsesickness virus than to bluetongue virus and epizootic haemorrhagic disease virus.