A nosocomial infection outbreak occurred in the Intensive Care Unit (ICU) of the Instituto Nacional de Pediatría (INP) in México City, during the months of March, April and May in 1988 Serratia marcescens was isolated as the etiological agent for this epidemic. Up to date, the source of contamination, the spreading and the pathogenic mechanisms which were involved in this outbreak remain unknown. In order to study the dynamics of the bacterial population involved in this outbreak, all strains of nosocomial S. marcescens isolated during 1988 were collected and studied. Eighty nosocomial strains were analysed. For this purpose we used four different markers: antibiotic susceptibility, presence of plasmids, exoenzyme production and pigment synthesis from a precursor. Using these markers, we were able to establish that five subpopulations of bacteria were present during the ICU outbreak, and that one of these subpopulations, VIII-A, was the most frequently isolated. A short time after this outbreak, we obtained S. marcescens isolates with similar properties which proceeded from other hospital units, suggesting intrahospital dissemination of the strain in question. We believe that, eventually, this study will allow us to establish bacterial spreading models within our institution.