Isolates were retrieved from drinking water and from animal and human sources associated with a waterborne outbreak of giardiasis. This is the first report of water-sourceand epidemic-associated Giardia isolates being adapted to in vitro propagation. Outbreak-associated, non-outbreak- associated, and reference isolates werecharacterized using isoenzyme electrophoresis and pulsed-fieldgel electrophoresis (PFGE). All outbreak-associatedand 2 other isolates were in one of eight zymodemes. The chromosomal complement of the outbreak-associated isolates was relatively homogeneous;this PFGE karyotype was distinguishable from other karyotypes.Overall results of both characterization methods were similar, although PFGE appears to be a more discriminating biotyping technique. Banding patterns of the outbreak-associated Giardia isolates remained the same even though the parasite passed through different hosts during the outbreak. Heterogeneity of isolates was also demonstrated for the first time within a singlecommunity not associated with the outbreak.