Over a period of one month, five isolates of Proteus mirabilis resistant to extended-spectrum cephalosporins colonized or infected five patients of the same intensive care unit. The antibiotic resistance phenotype and the metabolic profiles of the five strains were identical. All five produced a plasmid-mediated extended-spectrum β-lactamase closely related to TEM-3/CTX-1 according to its pI (6·3), to its substrate profile and to the hybridization of the corresponding gene with blaTEM-3 specific oligonucleotide probes. Ability to produce this enzyme transferred by conjugation to Escherichia coli along with aminoglycoside and sulphonamide resistance. These resistance markers were encoded by a plasmid, (pAP1) which was identical in all five isolates but which was different from the TEM-3 β-lactamase plasmids found in other Enterobacteriaceae. Plasmid pAPI differed by its smaller size (47 kb), its EcoRI restriction pattern, its resistance co-markers (AAC(6') and sulphonamide only) and its much lower transfer frequency (2×10−8). Furthermore this plasmid did not belong to any known Enterobacteriaceae incompatibility group.