Double-labelling experiments with a14C-labelled thiophosphoric acid ester (Hostathion®) and a solution of 18% saccharose mixed up with THO are described. The tests were performed onAulacorthum circumflexum (Buckt.) (Homoptera, Aphididae). The insecticide and the lipid were administered orally by the sachet-method. The concentration of the insecticide ranged from 0.1-0.0001%. The lowest absolute uptake of insecticide per aphid was 0.686·10-3 μg and the highest 1.389·10-3 μg; the absolute uptake of liquid per aphid ranged from 686·10-3 μl to 2.3·10-3 μl, but this figure takes no account of transpiration. The duration of action varied between 6.3 hours and 77.8 hours according to the concentration administered. The phases of intoxication are described; the duration of the phases increased with lower concentrations of insecticide.