DURING AN INVESTIGATION OF A MAJOR OUTBREAK OF GASTROENTERITIS IN THE DOUGLAS, WYOMING AREA ON JULY 27, 1992, SAMPLES WERE TAKEN FROM RESPONDENT'S PUBLIC WATER SYSTEM BY STATE AND LOCAL HEALTH OFFICIALS. TEST RESULTS OBTAINED BY THE WYOMING DEPARTMENT OF HEALTH AND SOCIAL SERVICES DEMONSTRATED THE PRESENCE OF FECAL COLIFORM BACTERIA, SPECIFICALLY SHIGELLA SONNEI, AN ORGANISM KNOWN TO CAUSE DISEASE. THE ORGANISM SHIGELLA SONNEI WAS IDENTIFIED AS THE CAUSE OF AT LEAST 150 CASES OF GASTROENTERITIS IN INDIVIDUALS ATTENDING A GATHERING AT AYRES NATURAL BRIDGE PARK. STATE AND LOCAL HEALTH OFFICIALS PROMPTLY CLOSED DOWN THE WATER SYSTEM TO PREVENT ADDITIONAL CASES OF SHIGELLOSIS.