Analysis of the nucleotide sequence of the genome segment S5 of Rice gall dwarf virus (RGDV), which belongs to the genus Phytoreovirus, revealed that this segment encodes a putative protein of approximately 91 kDa. Antiserum raised against the protein reacted with a minor structural protein of similar size in RGDV, demonstrating that S5 encodes the structural protein of RGDV. The protein named P5 appeared to be a component of viral core particles. Although no overall homology was found to any other proteins, including those of animal-infecting reoviruses, P5 of RGDV exhibited a significant homology to P5 of Rice dwarf virus (50%), which might be a guanylyltransferase, and to P5 of Wound tumor virus (55%). These results, together with the observation that P5 bound guanosine triphosphate suggest that P5 of RGDV shows a guanylyltransferase activity and, moreover, that plant-infecting reoviruses have a similar functional and/or structural organization to that of animal-infecting reoviruses.