Isoenzyme patterns of 32 isolates of Giardia duodenalis, obtained from 6 beavers and 11 humans from British Columbia, plus 15 other isolates were evaluated using thin-layer starch-gel electrophoresis. We attempted to use 12 enzymes; 9 gave reproducible and interpretable results. The isoenzyme patterns of the isolates were classified into 12 groups with 17 (53%) of the 32 isolates confined to 1 group. The other 11 groups each comprised only 1 or 2 isolates. There was no obvious correlation between clinical symptoms and isoenzyme patterns. Our findings suggest that beavers, like humans and gerbils are receptive to organisms with many different isoenzyme patterns.