The bacteriophage MAV1 is required for the development of arthritis in rats after infection with its host Mycoplasma arthritidis. To identify the phage-encoded virulence factor for this arthritis, the complete nucleotide sequence of MAV1 was determined. The linear double-stranded genome of MAV1 is 15,644 bp and contains 15 ORFs. Putative protein products from these ORFs were identified by comparison of the deduced amino acid sequences to known proteins and comprise DNA replication, restriction-modification, structural, regulatory, and integration/excision proteins. Eight putative promoters were identified; four of these would produce polycistronic transcripts. Translation of each ORF appears to be initiated independently, with each having its own RBS. A single ORF, vir, was identified on the minus strand of the phage genome. The putative protein product of vir contains a classic prokaryotic lipoprotein signal sequence and is a strong candidate for the MAV1-encoded virulence determinant.