The present invention relates to novel strains of avian reovirus that were isolated from clinical cases of viral arthritis/tenosynovitis in chickens in the southeast United States. The invention is directed to these novel group 1 and group 2 avian reoviruses, diagnostic assays using antibodies and/or nucleotide- or amino acid-specific components of such viruses, such as the S1 gene encoding the sigma C protein, and to vaccines that protect chickens from disease caused by such viruses.