A portable apparatus was used to filter large quantities of water from streams in California's Sierra Nevada. Samples were processed and examined forGiardiaspp. Cysts ofGiardiawere detected in 22 of 49 (44.9 percent) samples collected at sites downstream from areas of high recreational use and in 5 of 29 (17.2 percent) samples collected at sites downstream from areas of low recreational use. These data indicate that intensity of human use may play a significant role or be a useful indicator inGiardiacontamination of surface water.