Tests of DDT as a larvicide to control Simulium breeding in swift flowing mountain streams in Guatemala are reported. Using an emulsion containing 4% DDT made from a stock concentrate containing 20% DDT, 20% Triton X-100 and 60% xylene, complete eradication of Simulium larvae from streams for distances up to 10 km. was effected at concs, of 1 part DDT to 10 million parts water. Efforts to prolong larcivical action by absorbing emulsions and solns. of DDT on porous substances were unsuccessful. However, solns. of DDT absorbed by porous substances offer possibilities as a method of application for treatment of small streams. Trials of turpentine and crude saponin from soap berries indicate that they are moderately effective as solvent and emulsifying agent, respectively, and might economically replace xylene and Triton in Guatemala. Further tests show that DDT powder made into a suspension in water with a wetting agent gave kills as satisfactory as those obtained with emulsions.