Sarcocysts morphologically similar toSarcocystis muriswere observed in the skeletal muscles of mice dosed with free isosporan sporocysts recovered from the faeces of naturally infected cats. Kittens fed these mice shed similar sporocysts measuring 8.7 ±0.1 x 10.8 ±0.1 μm in their faeces five to six days later. Feeding these sporocysts to mice and rats resulted in the formation of identical sarcocysts in mice, but not rats. Attempts to induce similar infections in mice using naturally acquired isolates ofI. feliswere unsuccessful.