Comparative studies were carried out on the in vitro cultivation of Giardia duodenalis from dogs and humans. Cultures were initiated with trophozoites obtained by artificial excystation of cysts present in human or canine faecal specimens, or using trophozoites collected from the small intestine of dogs postmortem. 12 new human isolates of G. duodenalis were established in axenic culture from cysts present in faecal specimens, and successfully cryopreserved, an overall success rate for in vitro establishment of Giardia from cysts of approximately 44%. In contrast, not one of 24 canine isolates, whether of faecal or intestinal origin, became established in vitro. Since identical media and culture conditions were used for the cultivation of both human and canine isolates, the results may reflect strain differences. The zoonotic significance of such intraspecific variation is discussed.