Although reovirus infection is one of the major virus diseases of muscovy ducks in France, no vaccine is available and nothing is known about the structure and function of the genes and proteins of the reovirus involved. The complete S3 genome segment of the muscovy duck reovirus strain 89026 has been cloned and the nucleotide and deduced amino acid sequences are reported here. The S3 genome segment is 1201 bp long and possesses the same terminal motifs (5h GCTTTTT and TATTCATC 3h) as the S3 genome segment of known chicken reovirus strains. It contains one open reading frame that encodes a protein of 367 amino acids with a molecular mass of 40n8 kDa. The gene, encoding the σB major outer-capsid protein, was cloned into two different baculovirus transfer vectors and expressed in insect cells as a glutathione S-transferase fusion protein or a non-fused protein. The antigenicity of the two recombinant proteins was demonstrated by immunoblot assay. The potential immunogenic role of σB protein was studied in a protection assay against reovirus infection of specific-pathogen-free muscovy ducks. No antibodies could be detected by ELISA or immunoblot in ducks immunized with the recombinant proteins and no significant protection was noted after the challenge. However, whereas the weights of wild-type baculovirus-infected and challenge-control ducks were significantly lower than those of unchallenged ducks, the weights of male ducks previously immunized with the σB recombinant proteins did not differ significantly from males of either group. This work is the first to provide molecular data for a duck reovirus.