In feeding tests, 1.0\% thiotepa (tris (1-aziridinyl) phosphine sulfide) in 5\% sucrose solution was rapidly lethal to most females of the yellow-fever mosquito, Aedes aegypti (L.). At this concentration it was calculated that approximately 4600 mg of thiotepa per kg of mosquito weight was ingested.

The mortality of mosquitoes feeding on 0.1\% thiotepa was 69\% on day 20, 17 days after exposure, as compared with 23\% in untreated mosquitoes. With 0.1\% thiotepa, there was a significant reduction in the size of oocysts and obvious abnormalities in their appearance, but there was no significant difference between the numbers of oocysts in treated and untreated mosquitoes. Sporozoite production was delayed but not entirely prevented; the length of the extrinsic period being almost doubled. Dissections made on day 20 or thereafter indicate that the sporozoite rate was reduced by 68\%. Transmission to chicks by thiotepa-treated mosquitoes (on day 20) was reduced by 88\%. Plasmodium gallinaceum Brumpt in the 2 chicks infected by thiotepa-treated mosquitoes was infective to other mosquitoes and developed normally in them.

The effects of lower concentrations of thiotepa were studied in less detail. Sporozoite rates were reduced and the appearance of sporozoites was delayed in mosquitoes that fed on 0.01\% thiotepa but were unaffected in mosquitoes that fed on 0.001\% thiotepa.