Oocysts of Plasmodium relictum were cultured on stomachs of Culex tarsalis in a modified Grace's medium in the presence of Grace's cell line of Aedes aegypti. In vitro development of individual oocysts was approximately twice as great as in previous experiments where mosquito cell lines were not used. Oocysts 5 days old produced sporozoites in about the same length of time as occurred in vivo at the same temperature. The nature of the stimulus from the mosquito cells is being investigated. Over the past several years, we have been engaged in the cultivation of the mosquito phase of Plasmodium relictum. We have succeeded in obtaining the complete cycle in vitro from the stage of the zygote to that of infective sporozoites (Ball, 1964; Chao and Ball, 1964). However, it has not been possible to maintain a single preparation of the plasmodium for the entire period necessary for development in culture. Although the parasites may persist apparently unchanged in vitro for as long as 3 weeks (Ball, 1954), growth and development cease normally after 4 or 5 days, regardless of whether the plasmodium is grown in association with the mosquito stomach, or the oocyst is cultivated after being dissected away from the stomach (Ball and Chao, 1957). Consequently, it has been necessary to overlap cultures of the mosquito phase of P. relictum, with the result that four or five cultures have been required in order to obtain the complete cycle. The establishment of mosquito cell lines (Grace, 1966; Singh, 1967) offered the opportunity of employing them in an attempt to grow sporogonic stages of plasmodia. There was a good possibility that actively multiplying mosquito cells would supply the malaria parasite with some essential substances either absent from the culture media previously employed or else present in insufficient amounts for longterm development. Since only certain types of cells of the mosquito stomach remain viable for any length of time in whole organ cultures, the mosquito stomach, although persisting for a Received for publication 8 September 1970. * Supported by Grant AI-00087 from NIAID, U. S. Public Health Service, and Research Grant 254, Zoology, University of California. considerable period in vitro, may be unable to provide these essential substances. Wood and Suitor (1966) were the first to use mosquito cell lines with cultures of mosquitoborne parasites. Using Grace's A. aegypti cell line, they succeeded in obtaining development of the microfilariae of Macacanema formosana from the form found in the blood to secondand possibly third-stage larvae. Growth took place only in the presence of actively multiplying mosquito cells. Schneider (1968a) cultured P. gallinaceum in a modified Grace's mosquito medium, but without mosquito cells. If extracts of whole mosquitoes or of mosquito organs were added, 8to 9-day-old oocysts, in which sporoblasts were already present, developed into sporozoites. These retained their viability for 7 days, but were infective to birds for only 24 hr. In later experiments, Schneider (1968b), using oocysts of the same species in culture along with Grace's cell line of A. aegypti, found that the addition of the mosquito cells resulted in more rapid growth to maturity of 8-day oocysts if the A. aegypti cells were present in high concentration. In those mature oocysts which did not rupture spontaneously, the sporozoites remained active within the oocysts in vitro for 15 days. Seven-day oocysts developed immature sporozoites in 3 or 4 days if mosquito cells were prevented from overgrowing the parasite by DNA suppressors. Fink and Schicha (1969) used Grace's medium without mosquito cells to maintain sporozoites of P. cathemerium and of P. berghei yoelii. The latter species was maintained in an infective state for 6 hr. Wallicker and Robertson (1970) obtained development of 6to 9-day oocysts of P. berghei to infective sporozoites in Grace's medium supplemented with various