Within the group of protozoa the genus Giardia is included in the phylum Sarcomastigophora. In this genus different species are admitted, however, according to the disposition of the microtubular structures present in the middle bodies, three groups of species are admitted: Giardia agilis, Giardia muris and Giardia intestinalis (duodenalis or lamblia). This flagellated protozoan colonizes the intestine of humans and animals, producing a disease called Giardiosis that can be asymptomatic, acute or chronic, with diarrheal symptoms of varying intensity that can lead to malabsorption syndrome. Genotype A can be divided into at least three subgroups: the first (AI), detected in humans and animals, has a greater zoonotic potential; the second (AII), observed mainly in humans, and the third (AIII), apparently species specific, has been isolated from wild ungulates. Genotype B comprises two recognized subgroups, BIII and BIV, which have been mainly associated with humans, although there are also some reports in animals. However, compared to A, this genotype has a great polymorphism, which leads us to think about the existence of many more subtypes.