A plasmid containing multidrug resistance genes has been discovered from a clinical Actinobacillus pleuropneumoniae strain isolated in China. The complete 15079kb sequence of this plasmid, designated pHB0503, was analyzed with regard to the organization and evolution of multidrug resistance genes. The deduced amino acid sequences from seven open reading frames (sul2 catA3, aacC2, strA, truncated strB (strB'), bla(ROB-1) and aph(3')-I) identified in pHB0503 were entirely or nearly identical to resistance genes of plasmids both within and outside of the family Pasteurellaceaea, indicating that pHB0503 arose through inter-plasmid recombination processes among them. In addition, co-transcription of the cluster of resistance genes from the promoter upstream of sul2 and bla(ROB-1) was confirmed by RT-PCR. This is the first report of a complete sequence of the plasmid containing seven resistance genes from A. pleuropneumoniae.