Abstract 1. 1. The metabolism of P 32-labelled Di-syston R ( O, O-diethyl S-2(ethylthio)ethyl phosphorodithioate) in fifth instar bollworms, Heliothis zea (Boddie) from unentrained and light-synchronized populations was compared by the use of radiometric procedures. 2. 2. Larvae were treated by injection at consecutive 2-hr intervals for 24 hr and then analyzed to determine the concentrations of hydrolytic and oxidative metabolites of Di-syston R that were formed in 4 hr. 3. 3. Differences in metabolite concentrations in unentrained larvae were not significant; however, in entrained larvae variations were significantly different and followed a cyclic pattern. 4. 4. Maxima of hydrolysis product concentrations occurred at 24-hr intervals in internal extracts and at 12-hr intervals in excreta. 5. 5. Peak concentrations of oxidation products occurred at 12-hr intervals in internal extracts but did not follow a definite pattern in excreta.