The technique of allozyme electrophoresis was applied to 29 Australasian stocks and 48 clones of Giardia intestinalis from humans as a means of increasing the number of genetic markers currently available for identification and classification. Fifty different enzymes were examined and of these 26 loci were found to be suitable for use as genetic markers. The data indicate the presence of four discrete genetic groups within the sample of G. intestinalis examined. The groups had fixed genetic differences at 23-69% of loci established. The evidence suggests that G. intestinalis is a species complex. The results have important implications for the systematics of human isolates of Giardia, as well as for studies on the epidemiology and demography of giardiasis in Australia and elsewhere.