Interhospital spread appeared to be responsible for a large epidemic of infections due to a strain of Serratia marcescens that was resistant to all currently available parenteral antibiotics. Between April 1, 1973 and January 1, 1975, 210 patients in four geographically separate hospitals in Nashville, Tennessee, were infected with the epidemic strain; 21 patients were bacteremic and eight died. Catheter-associated urinary tract infection accounted for the majority of isolates, and broad-spectrum antibiotic exposure appeared to promote the acquisition of the epidemic strain. The serotype (O1:H7) and phage type (186) of the organism were identical in all four hospitals, but background, sensitive strains of S. marcesens yielded a variety of other serotypes. Carriage on the hands of hospital personnel was implicated as the mode of spread within the hospital and apparently was the mode of transmission between the hospitals. Antibiotic resistance was largely episomally mediated, but resistance to gentamicin, cephalothin, and colistin was not transferable.