Abstract: The laboratory animals were challenged with trophozoites of G. lamblia from axenic culture and trophozoites of G, simoni from naturally infected golden hamsters by oral and duodenal routes. The results showed that all the animals (9 rats,9 guinea pigs and 4 rabbits) challenged with cultured trophozoites of G. lamblia were not infected，and that when challenged with G. simoni the rats (n=10) were infected,while the guinea pigs ( n=8) and rabbits (n=2) were not. It indicated that there was a biological difference between G. lamblia and G. simoni although they were morphologically identical. It was also observed that young rats were more susceptible than adult rats to G. simoni. The cysts of G. simoni were detected in rat faeces. It indicated that G. simoni could complete its life cycle in rats.