Isoenzyme electrophoresis using 13 enzyme systems was applied to 31 Australian and 7 Swiss isolates of Giardia of human, cat, cattle, dog, sheep and rat origin. The Portland (ATCC No. 30888) reference strain was also included. The 39 isolates were divided into 22 different zymodemes. These consisted of 19 zymodemes containing the P1 and Australian isolates and three zymodemes containing Swiss isolates only. Differences in enzyme profiles between zymodemes was measured by euclidean distance and it was found that Australian isolates of Giardia exhibited more variation than the Swiss isolates. Relationships between zymodemes determined by clustering analysis are discussed with particular reference to the zoonotic potential of Giardia.