AbstractSamples of three commercial pyrethrum extracts were examined chemically and biologically to determine the number of insecticidal constituents they contain. These constituents, which were completely removed from commercial pyrethrum extract by exhaustive extraction with nitromethane, were separated by displacement chromatography. The insecticidal activity of the eluate was restricted to fractions identified chemically as cinerin I, pyrethrin I, cinerin II and pyrethrin II. Solutions containing the four pure active constituents in the same ratios as in the three commercial extracts had the same activity as the corresponding extracts. Pyrethrin II was more active than pyrethrin I but the cinerins were much less active. If any other insecticidal constituent occurs in commercial pyrethrum extract its contribution to the total activity is negligible.