A 62-kDa cell surface antigen (M9) of Mycoplasma gallisepticum PG31 that mediates antibody-induced agglutination of the organism was purified and subjected to N-terminal amino-acid sequencing. A 999-bp region of the cDNA encoding the M9 protein was generated by reverse transcription-PCR, and its nucleotide sequence was determined. PCR primers based on this sequence were used to screen a genomic DNA library of PG31. A full-length M9 protein- encoding gene was isolated and sequenced, revealing 96% nucleotide identity with the pMGA1.1 gene of M. gallisepticum S6. Sequence analyses of the M9 gene and flanking open reading frames that encode other pMGA family members suggest that a tandemly repeated GAA sequence may influence pMGA gene expression.