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Human giardiasis, the gastrointestinal infection caused by two genetically different groups (or assemblages) of Giardia duodenalis, is very common worldwide, and its prevalence is higher in developing countries. However, few surveys in these regions have been performed to include a genetic characterization of the parasite, which is necessary to unravel the complex epidemiology of the infection. In this work, we screened 120 faecal samples collected from Sahrawi children in 2003-2005, and found 41 (34.2%) of them to be positive, using immunofluorescent microscopy, for the presence of G. duodenalis cysts. Molecular characterization of the isolates was performed by RFLP and/or sequence analysis of the triose phosphate isomerase (tpi) and the glutamate dehydrogenase (gdh) genes. The results disclosed an unexpectedly high genetic polymorphism among isolates of both assemblages A and B, and a large percentage of the sequences (50% for the tpi gene, and 90% for the gdh gene) from assemblage B isolates characterized by the presence of overlapping nucleotide peaks at specific positions in the chromatograms, which can be attributed to mixed infections or to allelic sequence heterozygosity of single cysts. Notably, this phenomenon was not observed in sequences from assemblage A isolates. These results suggest that the genetic structure is different in isolates of assemblages A and B.