A Giardia infected Washington State child was found to participate in an infant and toddler swim class. A stool survey of 70 child participants revealed a 61 per cent prevalence of Giardia infection. Also, 39 per cent of 53 mothers and 28 per cent of 21 fathers were Giardia positive. None of the non-swimming playmates were positive. Thirty-five per cent of 23 children exposed only at a better maintained pool to which the classes had been moved four weeks prior to the survey were positive. No evidence of transmission to non-swim class pool users was found.