Fifteen cases of Aeromonas hydrophila septicaemia, characterized by their frequent pulmonary lesions and the severity of their course, are reported. The delay observed between admission to hospital and first symptoms, together with the presence of anatomical lesions or physiological disturbances suggesting bacterial invasion by the intestinal route, have led the authors to postulate a nosocomial digestive contamination, probably from water, as suggested by the particular ecology of this micro-organism. This hypothesis was supported by the finding of Aeromonas hydrophila in large numbers at the different points of the hospital water distribution system where samples were taken.