The complete nucleotide sequences encoding precursor polyprotein (VP2-VP3-VP4) and VP5 of a highly virulent (hv) infectious bursal disease virus (IBDV), UPM97/61 was determined. Comparison of the deduced amino acid sequences with the published ones revealed 8 common amino acid substitutions, which were found only in the hv IBDV including the UPM97/61 strain. Three of the amino acid substitutions (222 Ala, 256 Ile and 294 Ile) were used as a marker for determining hv IBDV strains. The other five substitutions (685 Asn, 715 Ser, 751 Asp, 990 Val and 1005 Ala) were also conserved in hv IBDV strains isolated in various countries. UPM97/61 strain demonstrated also 8 unique amino acid substitutions of which 3 were in VP2, 4 in VP3 and 1 in VP4. There was 1 unique amino acid substitution in VP5 at position 19 (Asp-->Gly) not found in other strains. However, all the strains have a conserved 49 Arg. The amino acid sequence of UPM97/61 strain differed by 1.09% from the Japanese (OKYM) and Hong Kong (HK46) strains, and by 1.48% from the Israeli (IBDVKS) and European (UK661) strains. Hence, UPM97/61 is more closely related to the hv strains from Asia. However, phylogenetic analysis indicated that the origin of UPM97/61 might be the same as that of other hv strains isolated from other parts of the world.