Developing oocysts of P. relictum were dissected off the stomachs of infected Culex tarsalis and were obtained in a sterile condition. Cultivated in a variety of media, they developed in vitro apart from the intact stomach of the mosquito vector. Development did not proceed beyond stages reached by the oocysts attached to an entire stomach which was not injured. The mosquito stomach is evidently not required for the development of oocysts of P. relictum.