The smaller dsRNA segment of the genome of infectious bursal disease virus (IBDV) encodes a single polypeptide of approximately 90 kDa (VP1). The consensus nucleotide sequence, derived from independent and overlapping cDNA clones, contains a single open reading frame which begins with an exact Kozak sequence and could encode a polypeptide of 878 amino acid residues. It has been suggested that VP1 could be the viral RNA-dependent RNA polymerase. A comparison of the predicted amino acid sequence of this protein with those of other DNA-dependent and ssRNA-dependent RNA polymerases has failed to reveal any homology between VP1 and the conserved regions in these enzymes. It is possible that the polypeptide encoded by the IBDV virus may represent a new class of polymerases which are involved in the replication of double-stranded RNA genomes.