Genetic properties and host ranges of R factors derived from Bordetella bronchiseptica of pig origin were examined. All of 61 R factors tested could confer resistance to streptomycin, sulfonamide, and aminobenzyl penicillin on their host bacteria. All of them were identified as fi − (no fertility inhibition) type and were found to exhibit no restriction of phages λ, φ80, P1, P2, T1, T3, T6, T7, W31, and BF-23. They could confer macarbomysin susceptibility on their host cells when infected. An Rte16, a representative R factor, was incompatible with both RP4 and R40a, which are classified as compatibility groups P and C, respectively. An Rte16 was conjugally transmissible to B. bronchiseptica, Escherichia coli, Citrobacter freundii, Salmonella typhimurium , and Yersinia enterocolitica , but not to Shigella flexneri, S. sonnei, Proteus mirabilis, P. vulgaris, P. rettgeri, Klebsiella pneumoniae , and Pseudomonas aeruginosa .