In November 1981, a waterborne giardiasis outbreak occurred within the Highlands Water and Sanitation District in Colorado. Although the immediate cause of the outbreak was reduced chlorine contact time because of a pump failure, numerous design and operational deficiencies existed at the plant. Lack of chemical pretreatment, improper backwashing procedures, poor application of raw water to the filter, and failure to monitor plant conditions all contributed to the outbreak. Turbidity and bacteriological records did not indicate any problems, confirming the crucial role of proper design, installation, and operation of the multibarrier technique in preventing waterborne giardiasis outbreaks. Giardiasis is an intestinal disease most