A cat fed commercially prepared cat food and free-living rats (Rattus rattus and R. norvegicus) passed coccidian oocysts which were morphologically similar to Toxoplasma gondii. Mice dosed with these oocysts did not develop toxoplasmosis, but instead developed small, thin-walled parasitic cysts containing small bradyzoites in their striated muscles. These cysts measured 35 to 90 micron by 9 to 40 micron and they resembled cysts of Hammondia hammondi. Unlike T. gondii this coccidian parasite was shown to have an obligatory two-host cycle and consequently it is considered to be H. hammondi. This represents the first recorded detection of H. hammondi in Australia. The source of the H. hammondi infection was a Rattus rattus. This represents the first record of a natural intermediate host for H. hammondi.