Macroscopically visible spherical cysts measuring up to 260/μm in diameter and showing characteristics typical of those of the genusBesnoitiawere observed in rats dosed with isosporan oocysts recovered from the faeces of a feral cat. Kittens fed these rats shed similar oocysts measuring 16.9 ± 0.1 × 14.6 ± 0.1 μm in their faeces 11–12days later. The feeding of sporulated oocysts of this coccidian to mice, rats, rabbits and guinea pigs resulted in the formation ofBesnoitiacystsin all hosts except the last.