The object of this study was to determine the importance of domestic cats in the epidemiology of human giardiasis. Of six laboratory-reared cats inoculated with cultured Giardia lamblia trophozoites from humans, only one showed the presence of cysts in the feces, and cysts were found on only 1 of the 80 days of observation. In a second experiment, eight cats were inoculated with G. lamblia cysts isolated from a human being. Over an 8-week period of observation, two of eight cats were found to have passed cysts in their feces, one on only one day and the other on 2 days. Postmortem examination of all of the cats found to be passing G. lamblia cysts at some time during the experiments did not reveal any small-intestinal trophozoites. These results suggest that domestic cats are relatively insusceptible to G. lamblia from humans and, consequently, that cats probably are not significant reservoir hosts of Giardia spp. infective for human beings. Moreover, it appears that the Giardia spp. which parasitize cats are distinct from those of human beings.