The plasmid profiles of 65 strains of Proteus penneri were analyzed to determine whether resistance was determined chromosomally or by plasmids. Only seven strains harboured one to three plasmids, although these strains exhibited resistance to a wide range of antibiotics. Markers for ampicillin and tetracycline resistance could be transferred to Escherichia coli by transformation. Plasmids carried resistance to chloramphenicol in two strains and resistance to sulfonamides in one strain. The result showed that resistance is determined chromosomally rather than by plasmids, however the possibility that these bacteria may acquire resistance plasmids which change their antibiotic susceptibility pattern cannot be excluded.