The DNA sequence of the gene encoding the early and specific 46-kDa surface antigen (P46) of Mycoplasma hyopneumoniae has been determined. The P46 gene, encoding a putative lipoprotein, contained three TGA codons and a single CGG codon in a 1,257-bp open reading frame. Edman degradation of peptide fragments showed that at least one TGA codon encodes tryptophan and that the CGG codon, which has been reported to be nonsense or unassigned in other mycoplasmas, is used for arginine in M. hyopneumoniae.