The olive scale, Parlatoria oleae, was used to test the increase in toxicity of petroleum oils due to the addition of various toxicants. Doses in terms of 10-6 ml. were measured in glass capillaries (about 0.05 mm. diam.) under a microscope. The M.L.D."s for light, medium, and heavy summer spray oils were 2.3, 1.8, and 1.6 X 10-6 ml. per sq. mm. of leaf area covered by the scale, respectively. Addition of 0.06% rotenone to the oils (by means of 11% by wt. of 3,5,5, trimethyl cyclo hexene -2-one-1 and 0.2% of derris resins containing 30% retenone) brought the M.L.D."s to approx. 0.7 in each case. Of the following materials, triamylamine, [beta]-thiocyano ethyl esters of fatty acids (Lethane 60) 1,1, dichloro 1 nitroethane, nicotine, 1 trichloro-2, 2-bis (p-chlorophenyl) ethane (DDT), methylnaphthalenes (Velsicol AR-60), ground cube root, and derris resins, the last 2 were markedly superior in increasing the toxicity of light summer petroleum spray oil.