Giardia duodenalis is an intestinal protozoal parasite capable of causing both clinical and subclinical disease in a broad range of species, including humans and cats. The parasite has a ubiquitous distribution and infection occurs worldwide in nearly all mammals. Giardia infection in cats may not be associated with any clinical signs, but affected cats most commonly exhibit acute small bowel diarrhea with or without weight loss. Estimates of the prevalence in cats fall within a broad range (between 1-10%), with higher rates generally reported for group housed cats. Confusion exists regarding the significance of Giardia in cats and the need for surveillance and treatment of cats in animal shelters as data regarding the zoonotic potential of Giardia-infected cats are lacking. Cysts of the various genotypes are morphologically indistinguishable by light microscopy and advanced molecular techniques are required to determine their assemblage type. Little information exists regarding the differences among the various assemblages, with the principal distinction being host range and possibly geographic location. Assemblages A and B have the widest host range and cause disease in humans. Assemblages C and D are found mainly in dogs, E in livestock, F in cats, and G in rodents. Mixed infections are possible. The purpose of this study was to estimate the prevalence of Giardia in cats available for adoption at an animal shelter in central New York, assess zoonotic risk by determining the assemblage type(s) found in cats shedding Giardia cysts, and determine risk factors for infection. Once monthly for 5 months a single fecal sample was collected from all cats available for adoption; samples were tested for the presence of Giardia antigen using a commercially available ELISA and all ELISA positive samples were processed for genotype analysis. There was significant variation in the overall prevalence of Giardia from month to month, with more than a two-fold increase between spring (April, May: 6.7%) and summer months (June, July, August: 13%). Stray adult cats and those with outdoor access were more likely to be infected than ownersurrendered cats and those that were kept indoors only. Among adult cats, increasing age was associated with decreasing risk of Giardia infection. Cats in colony housing, particularly at high densities and for prolonged periods of time, were also more likely to be shedding Giardia cysts. A total of 61 fecal samples that tested ELISA positive for Giardia were submitted for molecular analysis. Both zoonotic and host-adapted assemblages were identified; 75% of samples were the host-adapted assemblage F while 25% of samples (assemblages A and B) had zoonotic potential based on reported host ranges. Cats with potentially zoonotic assemblages were significantly more likely to be adults and to have soft stools or diarrhea than those cats infected with the host-adapted genotype. These results confirm that cats can not be discounted as a potential source of human Giardia infection, but identification of cats that pose a risk is not possible without advanced molecular diagnostics. Despite a greater understanding of the molecular epidemiology of the disease, the question as to whether or not 3 asymptomatic cats found to be shedding Giardia cysts should be treated in order to protect public health cannot be answered. Further work is needed with larger numbers of cats, both owned and from animal shelters, from various geographic locations to better understand the importance of various risk factors and to establish the true zoonotic potential of Giardia.