Water samples from three pristine rivers in the Pacific Northwest were analyzed for Giardia cyst presence and concentration using a membrane-filtration --immunofluorescence-assay procedure. A total of 222 samples were collected either monthly or bimonthly over a ninemonth period from 17 sampling stations on three rivers and 12 tributaries. Cyst recovery efficiency was monitored using samples seeded with cysts at levels ranging from 0.5 to 50 cysts/L. The recovery efficiency of the procedure averaged 21.8 percent +6 percent through 26 sets of samples. Giardia cysts were found in 94 (42 percent) of the samples. The corresponding cyst concentrations calculated from the recovery efficiency and the sample volume ranged from 0.1 to 5.2 cysts/L. The distribution of cyst concentrations in positive samples was lognormal. Both the magnitude of cyst concentrations (as indicated by the mean value) and the variability (as indicated by the slope of the distributions) differed among the three rivers. No statistically supportable seasonal variations were found. The principal conclusion was that Giardia cysts appear to be continuously present, though at low concentrations, even in relatively pristine rivers. Includes 12 references, tables, figures.