• An abrupt, severe outbreak of poliomyelitis occurred in an unusually homogeneous community under circumstances permitting an analysis of possible etiological factors. During a five-week period, more than 10% of the 347 children in the area of high incidence developed poliomyelitis; 4.6% of the children suffered paralytic disease, with death or with disability lasting at least two years. During this time, there were no cases of paralytic or nonparalytic disease among the 256 children residing in the adjoining section of the village. A detailed study of the water supply revealed a close spatial correspondence between the distribution of cases and the location of flush-valve water closets not provided with vacuum breakers, as well as a temporal correspondence between the outbreak and the occurrence of extreme fluctuations of pressure in the water mains. The distribution of cases differed strikingly from the randomness observed when measles and chickenpox spread in the same community. Immunological studies further corroborated the belief that type 1 poliomyelitis virus had been carried by contaminated drinking water. It is pointed out that the testing of water at a power plant is unlikely to reveal contamination originating nearer the outlets in the community.