An outbreak of Giardiasis was attributed to water at a private recreational camp in California. Two days before the outbreak began, the camp installed a slow-sand water filtration system that included unsterilized sand. There was a record of elevated coliform and turbidity levels in the sand-filtered spring water used for showering during the exposure period. Unfiltered spring water tested at the same time had acceptable coliform and turbidity levels, implicating the filtration system as the most likely contamination source.