AbstractThe resinous material formed by polymerization of a highly purified pyrethrum concentrate has been examined. It was insoluble in cold light petroleum or deodorized kerosene, and gave high values for apparent pyrethrins by the Seil or mercury‐reduction methods, although it was devoid of insecticidal effect on houseflies. In these respects it resembled the additional ‘pyrethrins’ recently shown1 to be extracted by chloroform or by warm light petroleum from pyrethrum flowers, differing only in its higher content of apparent ‘pyrethrin I’.On hydrolysis of this material, pure chrysanthemum mono‐ and di‐carboxylic acids were isolated in yields of some 75% of the quantities calculated from the analysis. These acids were identical with those derived from normal pyrethrum extracts. Hence it appears that polymerization is confined to the keto‐alcoholic constituents of the esters, the acids not being involved. The chemical assay methods at present available do not distinguish such polymerized material from true pyrethrins; but the modification described by Mitchell and Tresadern1 largely prevents the extraction of polymerized material.The need for confirming all chemical assays by biological tests is emphasized. A portion of the monocarboxylic acid was esteri‐fied with DL‐allylcyclopentenolone; the ester possessed the expected insecticidal activity. The action of this ester on houseflies is discussed and contrasted with that of the pyrethrins.