Over a two-month period there was a sudden increase in Aeromonas hydrophila isolated from colon i.e., biopsies from patients without related symptoms. Strains were studied by biotyping (API 20 NE, bioMerieux), antibiotyping, plasmid analysis, SDS-PAGE of whole cell proteins and toxicity for cell cultures. All strains gave identical results. In particular SDS-PAGE of whole cell proteins showed an identical electrophoretic pattern for all the strains, and so all were considered to be of the same clone. During the study period, endoscopic materials were disinfected with quaternary ammonia and glutaraldehyde phenate. After these disinfectants were changed to glutaraldehyde 2%, there were no more isolates of A. hydrophila from the biopsies. We conclude that SDS-PAGE can be a useful technique for epidemiological characterization of A. hydrophila.