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Summary.1 The toxicities to Aphis rumicis of certain aliphatic and aromatic amines and of some of the simpler nitrogen‐heterocyclic derivatives have been quantitatively determined. 2 Tetramethylammonium hydrate and chloride are more toxic than the corresponding tetraethylammonium compounds. This is in keeping with the findings of Dale and his co‐workers who have shown that tetramethylammonium has certain physiological effects similar to those of nicotine which are not shown by tetraethylammonium. 3 The aromatic amines, on the whole, show little insecticidal action. Aniline and most of the aliphatic anilines are only slightly toxic to A. rumicis . The substitution of aromatic groups in the amino group of aniline increases toxicity more than the substitution of aliphatic groups. There are interesting relationships in regard to toxicity among these compounds. For example, the following orders of toxicity were noted:Phenylamine (aniline) < diphenylamine > triphenylamine. Phenylamine (aniline) < benzylaniline > dibenzylaniline. Benzylamine < dibenzplamine > tribenzylamine.4 o ‐Nitraniline is one of the most toxic of the aniline derivatives. 5 α‐Naphthylamine is more toxic than aniline. Substitution of various radicals in the amino group of aniline has a greater effect on the toxicity of the resulting compound than substitution of the same radicals in α‐naphthylamine. α‐Naphthylamine derivatives are more toxic than the corresponding β‐derivatives. 6 Among the heterocyclic compounds, nicotine is highly poisonous to A. rumicis . The heterocyclic rings constituting the molecule of nicotine are much less toxic than nicotine itself; pyrrole and pyridine show comparatively slight insecticidal action. The order of toxicity of the simpler N‐heterocyclic compounds runs:Pyrrole < pyridine < picoline < lutidine < quinoline and isoquinoline < acridine.7 Hydrogenation of pyridine and pyrrole increases their toxicity; piperidine is more toxic than pyridine and pyrrolidine than pyrrole. 8 Benzyl‐pyridine is the most toxic pyridine derivative tested.