Thirty-three epidemiologically unrelated strains of ampicillin-chloramphenicol-resistant isolates of Haemophilus influenzae (22 type b, 11 unencapsulated), isolated over 10 years in Belgium, were compared with 53 ampicillin-resistant chloramphenicol-susceptible isolates (22 type b, 31 unencapsulated). All ampicillin-chloramphenicol-resistant and 76% of ampicillin-resistant chloramphenicol-susceptible strains were resistant to tetracycline, kanamycin, or both. Resistance to these antibiotics was specified by a 37- to 44-MDa conjugative plasmid. The genetic relatedness of these plasmids and of those in multiresistance strains from Spain was investigated. Plasmids specifying ampicillin-chloramphenicol-tetracycline-kanamycin resistance in Belgium or in Spain had highly related restriction fragment patterns. By homoduplex analysis, they had similar molecular organization and contained a structure identical to Tn10-TnCm, a transposon previously identified in chloramphenicol-tetracycline-resistant H. influenzae. Plasmids coding for different resistance phenotypes had less resemblance by restriction endonuclease analysis; however, study of heteroduplex molecules indicated they shared a high proportion of core sequences. These findings support the hypothesis of independent transposition events resulting in resistance plasmids of close molecular organization.