The genetic basis of drug-resistant strains of Actinobacillus pleuropneumoniae in Japan was studied. The A pleuropneumoniae strains AV277 and AV281 that belong to serotype 2 were resistant to streptomycin (SM) and sulfonamide (SA). Both strains had an 8.1-kilobase (kb) SM-SA plasmid that was previously classified in the H1 group. The AV177 (serotype 1) strain was resistant to SM, SA, ampicillin, and kanamycin (Km), but did not have any plasmids. The AV319 and AV324 (serotype 1) strains were resistant to Sm, SA, tetracycline (TC), and chloramphenicol (CP). The AV318 (serotype 12) strain was resistant to SM, SA, TC, minocycline, and CP. These 3 strains (AV319, AV324, and AV318) had a 4.3-kb SM-SA plasmid and a 5.2-kb CP plasmid. The 4.3-kb plasmid was classified in the H2 group. The AV263 (serotype 1) strain was resistant to SM, SA, KM, TC, and CP. It had a 5.2-kb CP plasmid and a 6.6-kb SM-SA-KM plasmid. Both plasmids did not replicate stably in Escherichia coli strains. The former 5.2-kb plasmid was mobilized in E coli strains by plasmid RP4, which belonged to incompatibility P with broad host range, but the latter 6.6-kb plasmid was not so mobilized. Three 5.2-kb CP plasmids isolated from strains AV319, AV324, and AV318, had the same restriction endonuclease pattern after digestion with Ava I and EcoRI. They coexisted with H1 group plasmids in the incompatibility test, and coexisted also with H2 group plasmids of the original A pleuropneumoniae strains. Results indicated that the 5.2-kb CP plasmids could be classified in a new incompatibility group, H3. In this study, 4 types of plasmids were isolated, but no plasmids encoded TC and minocycline resistance.