Neospora caninum is an apicomplexan parasite that causes neonatal neuromuscular disease in dogs and abortions in cattle. Bovine neosporosis is a major production problem worldwide. The parasite is transmitted to cattle via oocysts excreted by dogs or by transplacental transmission. Dogs are the only proven definitive host for N. caninum. One of 3 dogs fed mouse brains containing tissue cysts of a wild-type N. caninum strain CK0160SC3B (CKO) excreted oocysts in its feces. Two of 3 dogs fed mouse brains containing tissue cysts from a cloned line of the CKO strain excreted N. caninum oocysts in their feces. The results indicate that a single N. caninum tachyzoite contains all the genetic information needed to produce the asexual and sexual cycles in the canine intestine.