Examination of a series of isolates of Providencia stuartii collected over an 18 month period from a chronic-care patient at Bristol Royal Infirmary revealed the emergence of resistance to carbenicillin. Resistance was mediated by a 47 kb plasmid which transferred by conjugation to a plasmid-free strain of P. stuartii but not to Escherichia coli. Carbenicillin-sensitive isolates were either plasmid-free or contained a 36 kb cryptic plasmid. Restriction endonuclease mapping of this plasmid showed it to be closely related to 32 kb and 34 kb cryptic plasmids reported previously in P. stuartii from Bristol. Mapping of the R plasmid showed it to be derived from the 34 kb cryptic plasmid by transposition of two copies of Tn1.