Computer-assisted analysis of the putative polypeptide products encoded by the two open reading frames present in a large virus-like double-stranded RNA, L-dsRNA, associated with hypovirulence of the chestnut blight fungus, Cryphonectria parasitica, revealed five distinct domains with significant sequence similarity to previously described conserved domains within plant potyvirus-encoded polyproteins. These included the putative RNA-dependent RNA polymerase, RNA helicase, two papain-like cysteine proteases related to the potyvirus helper-component protease, and a cysteine-rich domain of unknown function similar to the N-terminal portion of the potyvirus helper-component protein. Phylogenetic trees derived from the alignment of the polymerase domains of L-dsRNA, a subset of positive-stranded RNA viruses, and double-stranded RNA viruses, using three independent algorithms, suggested that the hypovirulence-associated dsRNA and potyvirus genomes share a common ancestry. However, comparison of the organization of the conserved domains within the encoded polyproteins of the respective viruses indicated that the proposed subsequent evolution involved extensive genome rearrangement.