Extensive use of DDT in controlling disease-carrying arthropods has suggested the evaluation of its toxicity to other invertebrates which are of concern to man. Laboratory in vitro studies have demonstrated that DDT applied as a powder, in oils (xylene, No. 2 Diesel, kerosene, lubricating, and mineral) or in xylene-Triton (X-100) emulsions is not sufficiently cercaricidal to be of practical value in the control of schistosomiasis. Cercariae of S. mansoni, although severely affected and partially immobilized by xylene-Triton emulsion with and without DDT, are potentially infectious schistosomes until near death. A small but dangerous percentage of cercariae treated in vitro with DDT emulsions and then placed in untreated, fresh water will revive sufficiently to infect hamsters.