Previous outbreaks caused by Serratia marcescens have been associated with contaminated medical equipment, intravenous fluids and inadequate hygiene. We carried out the molecular characterization of an outbreak produced by a cephalosporin-resistant S. marscescens that occurred in a Mexican hospital in August 1999. The lethality of this outbreak was 26%. Positive isolates were collected from 20 patients, one medical staff and three chlorhexidine disinfectant solutions. Results of PFGE, beta-lactamase patterns, sequencing of PCR amplifications, plasmid profiles, and mating experiments showed that the outbreak occurred by the dissemination of a S. marcescens SHV-5 producing strain. The adequate enforcement of procedures under the supervision of an infection control resulted in the abrupt end of the outbreak.