A number of antibiotics, including some known to be toxic to and to suppress egg laying in orchard mites, were tested as sprays at different concentrations for toxicity and sterilizing effects on the green peach aphid, Myzus persicae (Sulzer), the apple aphid, Aphis pomi DeGeer, and the pea aphid, Macrosiphum pisi (Harris). In tests on the green peach aphid, cytovirin (from Streptomyces sp.), cycloheximide (3-[2-(3,5-dimethyl-2-oxycyclohexyl)-2-hydroxyethyl]glutarimide), streptovitacin A (3-[2-hydroxy-2-(5-hydroxy-3,5-dimethyl-2-oxocyclohexyl) ethyl] glutarimide), antimycin A (7-hexyl-3-(2-hydroxy-3-foramidobenzamido)-4,9-dimethyl-1,5-dioxa-2,6-dioxo-8-(iso-valeroyloxy) cyclononane), and hygromycin B (from Streptomyces hygroscopicus (Jansen) Waksman & Henrici) were definitely insecticidal. The other compounds were only slightly toxic or had no effect. Plssible sterilant effects of the more active antibiotics were not shown because of their toxicity and those with lower or no toxicity did not show any marked sterilant effects although the number of progeny was generally less than in the untreated control. Similar effects of some of these antibiotics were shown in tests on the apple aphid and the pea aphid.