1. An incomplete field study in Lincolnshire, England, suggested that toxoplasmosis might be transmitted from pig to man by mosquitoes. Although the results were inconclusive they suggested the experimental work recorded in this paper.
2. Trophozoites of the Beverley strain were obtained in increased yield from the peritoneal fluid of Mastomys treated with hydrocortisone. Cysts were obtained in increased yield by passage of the brain of Sigmodon into laboratory mice.
3. Four species of mosquitoes were fed through a membrane or on sucrose solution on media containing either cysts or trophozoites of Toxoplasma gondii. In some experiments blood was added to ensure passage of the feed to the mosquito stomach, in others it was excluded so that at least part would pass to the crop.
4. A spurious ‘cyst’ resembling but distinguishable from Toxoplasma was noted in the homogenate of mouse brain.
5. In no instance did mosquitoes retain infectivity as demonstrated by injection into mice, beyond the third day, but since experiments were not carried on beyond the fourteenth day the existence of an occult form cannot be excluded.