Laboratory and field expts. on mosquitoes were conducted with various concs. of DDT in the form of colloidal dispersions, oil emulsions and dusts. Under laboratory conditions colloidal solns. and emulsions of DDT show very high toxicity to larvae of Culex pipiens and Aedes sollicitans but low toxicity to pupae of the same spp. The toxicity to pupae can be materially increased by incorporating DDT into a mosquito oil emulsion. In dust form, DDT proved ineffective against pupae and of considerably lower toxicity to larvae of sub-surface feeding spp. than either in oil emulsion or in colloidal solns. DDT sprays were found toxic to fish, water snakes, toads, turtle, and many species of aquatic insects, but non-injurious to vegetation. In general, it appears that DDT may offer a new and highly potent weapon to supplement our present chemicals used in the control of mosquitoes.