Serratia marcescens, long considered a non-pathogen, is now found to be responsible for outbreaks of nosocomial infections. An outbreak of Serratia infection at 2 institutions is reported, in which 253 cultures of Serratia were grown and 115 patients were involved. The 3 most important conditions that preceded isolation of Serratia were the use of indwelling urethral catheters, antibiotic therapy and operation. All infections were acquired in the hospital. An epidemiological survey showed that the organism is present in the environment, even in the absence of active infection.