Radioactive allethrin labeled with tritium has been synthesized because it was needed as a model compound in studies of interest. It was prepared by a catalytic exchange process producing a material that was approximately 50% pure containing several breakdown products. Portions of about 20 mg each were purified on silica gel layers yielding 14 mg of [ 3H]allethrin, specific activity 61 mCi/mmol. Its purity, chemical and radiochemical, as determined by proven methods was 97 ± 1% m/m.