In 1982, Giardia lamblia cysts were found in the faeces of eight of 97 children (two to five years old) in day-care centres in Guelph, Ontario, Canada. None of the infected children showed any of the classical signs of giardiasis. The survey was repeated in 1983 in the Kitchner-Waterloo area, Ontario. In this survey nine of 147 children were infected with G. lamblia. Experimentally established Giardia-free colonies of mice, hamsters, rats, cats (adults and kittens) and dogs (adults and puppies) could not be infected with Giardia cysts from clinical and non-clinical patients. Also, kittens, hamsters, and mice could not be infected by trophozoites from an axenic culture. However, five-day-old suckling rats can be infected with G. lamblia. Also, the Giardia-free rats and dogs are susceptible to their own Giardia (G. simoni and G. canis respectively). This proves that cats and dogs are not reservoirs of G. lamblia and consequently are not responsible for the spread of the infection in the human population.