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In the fall of 1985, an outbreak of giardiasis occurred among several swimming groups at an indoor pool in northeast New Jersey. Nine clinical cases were identified, eight of whom had Giardia positive stool specimens. All were female; seven were adults (greater than 18 years) and two were children. The attack rate was highest (39 per cent, 5/13) for the ladies lap group who had exposure on one day. These cases had no direct contact with children or other risk factors for acquiring Giardia. Infection most likely occurred following the ingestion of swimming pool water contaminated with Giardia cysts. The source of Giardia contamination was a handicapped child who had a fecal accident in the pool. He was a member of a group that swam at the same time as the ladies lap group. A stool survey of the handicapped group showed that of the 20 persons tested, nine were positive for Giardia, including the specimen from this child. Examination of the pool records showed that no chlorine levels had been taken on the day of the fecal accident and that on the following day the chlorine level was zero. This is the second report of Giardia transmission among swimming pool attendees. It emphasizes the need to maintain appropriate chlorine levels in swimming pools and to institute measures to clear pools after a fecal accident.