The test substances were mixed into a nutritional medium consisting of ground lean beef, bovine blood, water, and sufficient formaldehyde to retard putrefaction. Newly hatched larvae of Cochliomyia americana were placed on the treated medium. Organic compounds were evaluated on the basis of the lowest concs. that produce 100% mortality within 3 days. The various substances were tested at concs. in the nutritional medium of 0.67, 0.33, 0.17, 0.10, 0.08, 0.05, 0.03, 0.01, and 0.005%. The following compounds were found to be toxic at concs. of 0.67% or less: acridine, p-dibenzodioxin, dibenzothiophene, dibenzyl, 5,10-dihydroacridine, 9,10-di-hydroanthracene, diphenyl, diphenylamine, diphenylene oxide, diphenyl methane, diphenyl sulfoxide, phenazine, phenothiazine, phenothioxin, phenoxazine, phenyl ether, and phenyl sulfide. Compounds non-toxic at a conc. of 0.67% were anthracene, carbazole, diphenyl sulfone, phenothiazine sulfoxide, and thianthrene. The relationship between mol. structure and toxicity to screwworm larvae is discussed. Any alteration of the molecule may influence toxicity.