A diabetic patient treated by continuous ambulatory peritoneal dialysis for end-stage renal failure had recurrent peritonitis caused by Serratia marcescens. Thirty-eight isolates of S. marcescens recovered over a 14-month period from peritoneal-dialysis effluent, catheter tips and catheter-exit sites were biotyped and serotyped. The finding that most (90%) of these isolates belonged to the same biotype and serotype suggested that the patient had relapsing infections with the same strain. Similar isolates were recovered from the peritoneal dialysates of another two patients in the same ward, neither of whom developed Serratia-associated peritonitis.