PCR/RFLP analysis to detect and genotype Giardia duodenalis on the basis of polymorphism in the triose phosphate isomerase (tpi) gene were applied to samples from 723 human giardiasis cases collected in England and Wales during 2001 to 2002. In addition, enhanced surveillance data was also collected from the patients using a standardized questionnaire. Among the 723 cases, the tpi gene was amplified from 537 (74%) of DNA samples extracted directly from whole faeces. Among these samples: one sample contained Assemblage A group I; 107 (15%) Assemblage A group II; 5 samples contained Assemblage A which could not be identified further; 419 (58%) Assemblage B; and, 5 (1%) a mixture of Assemblage A and B. The tpi gene could not be amplified from the remaining 186 (26%) of samples. Differences were detected in the distribution of the genotypes between regions within England and Wales. Includes 6 references, figures.