RECENT work1 has indicated that mevalonic acid (I) is an intermediate in the biosynthesis of terpenoid compounds. We now wish to report some experiments which indicate that this acid is also involved in the biosynthesis of the ‘pyrethrins’, the insecticidal constituents of the pyrethrum plant, Chrysanthemum cinerariaefolium. The ‘pyrethrins’ are a mixture of four closely related esters2 derived from two acids, chrysanthemum monocarboxylic acid (II, R = —Me) and pyrethric acid (II, R = —COOMe), and two keto-alcohols, pyrethrolone (III, R = —CH = CH2) and cinerolone (III, R = —CH3). It is customary to refer to the mixture of the two esters derived from chrysanthemum monocarboxylic acid and the keto-alcohols as ‘pyrethrin I’ and to that from pyrethric acid as ‘pyrethrin II’. The two acids have carbon skeletons which can be built up from two isoprene units, and thus might be expected to be derived from mevalonic acid. The two keto-alcohols cannot be so regarded, but it is of interest to note that the methylpentenone ring could be formed in theory by the cyclization of mevalonic acid. We have now shown that this theoretically attractive route is not employed by the plant.