The results of an investigation into the relation between chemical constitution and the knock‐down activity and toxicity of thiocyanate insecticides are recorded. Linkage of the toxic thiocyano radical through a ketomethylene group to a lipoid soluble hydrocarbon residue gives rise to knock‐down activity. The most active α‐thiocyanoketonesR.CO.CH2.SCN and thiocyanoacetatesR.O. CO. CH2. SCN are too irritant to the eyes and nose for inclusion in domestic fly‐sprays. Thiocyanoacetates derived from saturated 10‐carbon alcohols, however, are non‐irritant to the mucous membranes while possessing moderately high activity. The mode of action of these compounds