The acute toxicity of Diazinon®(0,0-diethyl 0-(2-isopropyl- 4-methyl-6-pyrimidinyl) phosphorothioate), Dipterex® (0,0- dimethyI2,2,2-trichloro-1-hydroxyethylphosphonate), Dow ET- 14 (0,0-dimethyl 0-2,4,5-trichlorophenyl phosphorothioate), Dow ET-15 (0-methyl 0-(2,4,5-trichlorophenyl)phosphoramido-thioate), malathion, and phenothiazine to 1-week-old chicks and thc toxic effect of the droppings from these treated chicks on 1- day-old house fly (Musca domestica (L.)) larvae were investigated. The LD-50, 7 days after treatment, in terms of mg. of technical toxicant per kg. of chick weight follows: Dinzinon, 8.4; Dipterex, 65; malathion, 370; Dow ET-14, 890; and Dow BT-15, 1,180. Phenothiazine was nontoxic at the highest level that it was possible to feed accurately to the chicks (1,970 mg./kg.). The effect of treatment on chick weight gain was also studied. The degree of fecal toxicity to house fly larvae was directly related to the dosage of insecticide ingested by the chick and inversely related to the period of time after ingestion that the feces were voided. Feces collected daily from chicks fed nonlethal levels of insecticides caused greater than 90% mortality in house fly larvae for varying periods of time, as follows: malathion, 1 day; Dipterex, 2 days; Dow ET-14, g days; and Dow ET-15, 6 days. Feces from Diazinon- and phenothiazine-treated chicks caused relatively low mortality in the larvae.