Summary In an endeavor to bring about the development of the sporogonic cycle of Plasmodium in vitro, attempts have been made to culture the oocysts of P. relictum, strain 1P1-1, on the stomachs of Cules tarsalis. A number of media, including completely synthetic ones were tested. Various mixtures of gases were introduced into the flasks, in some cases continuous aeration was employed, in others a modified rocker type culture was used. Mosquito tissue remained alive with hind-gut contraction for at least seven days; oocysts of P. relictum persisted apparently unaltered for six days, but no development of the oocysts could be demonstrated.