1. An explosive outbreak of gastroenteritis that affected 40–50% of people in a town of 10,800 inhabitants (Montrose) is described.
2. There is epidemiological evidence that the outbreak was water-borne. The chlorination of the water supply was faulty at the time of the outbreak.
3. Echovirus type 30 and two types of Coxsackie B viruses were isolated from sixteen out of thirty-two patients examined.
4. Shigella sonnei was isolated from the faeces of 110 out of 262 patients and contacts examined. Fifty-six strains tested for colicine activity were all colicine type 4 and had the same antibiotic sensitivity pattern.
5. This particular strain of Shigella sonnei (Montrose strain) spread to surrounding areas, although it disappeared quickly from Montrose.
6. The problem of inactivating virus in water supplies contaminated with sewage is discussed briefly.
We wish to acknowledge the help given by the general practitioners of Montrose and the staff of the Public Health Department of the County of Angus in the collection of specimens and information during the outbreak. We would also like to thank the Laboratory staffs of the diagnostic laboratory and virus laboratory of the Bacteriology Department, University of Dundee and the staff of the laboratory, Stracathro Hospital, for invaluable assistance. We are indebted to Dr R. R. Gillies, Epidemiology Unit, Bacteriology Department, University of Edinburgh, who kindly carried out colicine typing for us and supplied us with strains from his collection, and to Dr R. M. Gordon, Senior Medical Officer, Scottish Home and Health Department and Mr H. F. Scrimgeour, M.I.C.E., A.I.W.E., Engineering Inspector Scottish Development Department, for their assistance during the investigation.