Three cases of Aeromonas hydrophila septicaemia are reported. The first case concerned a 51 years old patient suffering from drowning who had previously been in good health and who died after a few days of refractory hypoxaemia. The second case was an 8 years old child with trisomy 21 operated for a congenital cardiac defect. The portal of entry was a catheter and the septicaemia was complicated by A. hydrophila mediastinitis, a complication which has not been described previously. The third case was a 75 years old man with no previous medical history, whose hobby was gardening. He developed septicaemia after a flu-like illness. A. hydrophila is a mobile Gram negative bacillus whose natural habitat is water. It appears to be an exceptional and temporary contaminant of the human G I tract. Clinical infection is rare and usually focal: gastroenteritis, infection of soft tissues after trauma in aquatic surroundings. In 60 out of 82 cases there was a documented deficiency in the patient's immune defenses. A digestive portal of entry was suspected in 63 cases but only rarely proved. In one of our cases the portal of entry was septic thrombophlebitis induced by an indwelling catheter; this complication has not previously been reported. The clinical picture of A. hydrophila septicaemia is characterised by the relative high incidence of Echtyma gangrenosum. The lethal outcome in 50 p. 100 of cases is often related to the patient's general condition. The sensitivity of the three cultures isolated was tested against third generation cephalosporins: they were effective in each occasion.