The infectivity of Giardia isolates in Mongolian gerbils and in SCID mice inoculated with a new established G. intestinalis strains of human origin (ISO/90/1, ISO/90/2) and a reference isolate ATCC 30888 has been investigated. Inoculation of Mongolian gerbils with trophozoites resulted in an acute and reproducible pattern of infection. 36 % of inoculated gerbils showed symptoms of infection after a first inoculation, whereas a second inoculation resulted in infection of 85% of the same gerbils. The group of SCID mice infected with ISO/90/2 failing to release cysts in the faecal material, while the second inoculated group infected with ISO/90/1 showed a low number of cysts at 13 days post inoculation. Cyst excretion was intermittent mostly released during the second and the third week of infection. Mongolian gerbils are useful to assess viability and infectivity of parasites from various sources. Strain differences may contribute to the development of different patterns of infections, which rely to clinical consequences in the diagnosis. Includes tables, figures.