Samples of DNA from a panel of Giardia isolated from humans and animals in Europe and shown previously to consist of 2 major genotypes--'Polish' and 'Belgian'--have been compared with human-derived Australian isolates chosen to represent distinct genotypes (genetic groups I-IV) defined previously by allozymic analysis. Homologous 0.52 kilobase (kb) segments of 2 trophozoite surface protein genes (tsa417 and tsp11, both present in isolates belonging to genetic groups I and II) and a 1.2 kb segment of the glutamate dehydrogenase (gdh) gene were amplified by the polymerase chain reaction (PCR) and examined for restriction fragment length polymorphisms (RFLPs). Of 21 'Polish' isolates that were tested, all yielded tsa417-like and tsp11-like PCR products that are characteristic of genetic groups I or II (15 and 6 isolates respectively) in a distinct assemblage of G. intestinalis from Australia (Assemblage A). Conversely, most of the 19 'Belgian' isolates resembled a second assemblage of genotypes defined in Australia (Assemblage B) which contains genetic groups III and IV. RFLP analysis of gdh amplification products showed also that 'Polish' isolates were equivalent to Australian Assemblage A isolates (this analysis does not distinguish between genetic groups I and II) and that 'Belgian' isolates were equivalent to Australian Assemblage B isolates. Comparison of nucleotide sequences determined for a 690 base-pair portion of the gdh PCR products revealed > or = 99.0% identity between group I and group II (Assemblage A/'Polish') genotypes, 88.3-89.7% identity between Assemblage A and Assemblage B genotypes, and > or = 98.4% identity between various Assemblage B/'Belgian' genotypes. The results confirm that the G. duodenalis isolates examined in this study (inclusive of G. intestinalis from humans) can be divided into 2 major genetic clusters: Assemblage A (= 'Polish' genotype) containing allozymically defined groups I and II, and Assemblage B (= 'Belgian' genotype) containing allozymically defined groups III and IV and other related genotypes.