This study was conducted to assess a dog's ability to differentiate between nematode-infected and uninfected sheep feces. Two German shepherd bitches were trained for scent detection over a 6-month period using operant/clicker conditioning. On completion of the training, testing was undertaken with 9 paper bags containing uninfected and 1 with infected feces, placed randomly around a circle. The dog and handler were not able to observe the placement of the bags. The 10th bag contained feces from sheep infected with either Teladorsagia circumcincta, Trichostrongylus vitrinus, Haemonchus contortus, or a mixed infection of all 3 species. Over 80 trials the dog had a mean success rate of greater than 80% in the detection of T. circumcincta- or T. vitrinus-infected feces and H. contortus-infected feces was detected with a slightly lower reliability of 76%, but mixed infections were detected at 92% reliability (one-proportion binomial analysis, P < 0.05). Trials were then undertaken to determine the time after administration of infective larvae that the dog was first able to differentiate T. circumcincta infection in sheep feces. At 7 days postinfection (dpi), the dog was capable of identifying T. circumcincta at least 85% of the time. These trials demonstrate that dogs are able to detect the common parasite infections in sheep with reliability equal to immunological assays. The results suggest that diagnostics based on odor detection using dogs to identify signature odors which could then be used to create sensitive detection devices might allow parasite detection on-farm and on all sheep in a flock.