Duodenal aspirates from children investigated for diarrhoea have been examined for the presence of Giardia over an eleven year period, and where possible, in vitro or in vivo Giardia cultures in mice were established. Based on biochemical characteristics of electrophoretic karyotype, RFLP analysis and rDNA hybridization studies of 40 stocks at least two major varieties, or demes, of Giardia have infected the population of South East Queensland and environs during this period. These demes carried different rDNA repeat units and differed markedly in both the electrophoretic karyotype pattern and the molar representation of chromosome bands. From 1983 to 1991 only one deme was documented. The first evidence of a new deme seen in local children occurred in 1991 and was followed by a predominance of this deme in 1993. These 40 stocks do not represent all positive samples. Other stocks established in vivo were not able to be cultured in vitro, and these probably represent other demes. Since all of the stocks established in vivo were not able to be cultured in vitro, and these probably represent other demes. Since all of the stocks were derived from children with similar chronic symptoms it appears that at least two demes of Giardia are pathogenic.