The prevalence of Giardia sp. in a beaver (Castor canadensis) colony in Colorado was determined by the collection and analysis of fecal samples over a period of 14 mo. Environmental contamination was monitored through the use and analysis of water filter samples. Beaver shed cysts of Giardia sp. in their feces throughout the year with temporal variations in the prevalence, and became infected as kits and remained infected as juveniles and adults. Beaver served as amplification hosts for Giardia sp. and contaminated surface waters downstream from their dams in late spring and early fall. In slow moving waters the cysts of Giardia sp. settled rapidly. Muskrats (Ondatra zibethicus) were the only other species of wildlife shedding cysts of Giardia sp. on the study area.