Well water served to visitors to an Island Park, Idaho resort led to an outbreak of Shigella sonnei. Samples of water tested failed to show the presence of Shigella, however the water had a high coliform bacteria count. A recent subdivision of homes had been built near the resort. Shigella sonnei was found in the stool of six persons who resided in these homes. These persons denied visiting the resort, but used well water at home. The water table was substantially higher than normal due to rainfall. Drainage from a sewer line for the subdivision was draining improperly.