AbstractThe residues and metabolism of methidathion [S‐(2, 3‐dihydro‐5‐methoxy‐2‐oxo‐1, 3, 4‐thiadiazol‐3‐ylmethyl)O, O‐dimethyl phosphorodithioate] and its secondary metabolites: demethyl‐methidathion [S‐(2, 3‐dihydro‐5‐methoxy‐2‐oxo‐1, 3, 4‐thiadiazol‐3‐ylmethyl)O‐methylO‐hydrogen phosphorodithioate] (IV), the sulphide (2,3‐dihydro‐5‐methoxy‐3‐methylthiomethyl‐1,3,4‐thiadiazol‐2‐one) (I), tsulphoxide(2,3‐dihydro‐5‐methoxy‐3‐ methylsulphinylmethyl‐1,3,4‐thiadiazol‐2‐one) (II) and the sulphone (2,3‐dihydro‐5‐methoxy‐3‐methylsulphonylmethyl‐1,3,4‐thiadiazol‐2‐one (III) were studied in laboratory‐treated tomato fruit. The metabolites and residues of methidathion were determined for the applied doses of 1, 7 and 14 mg of methidathion kg−1of fruit. Methidathion was metabolised extensively over a 14‐day period. The amount of metabolites formed was a function of both the applied dose as well as the time after application. Major water‐soluble metabolites were found to beIVand the cysteine conjugateS‐(2,3‐dihydro‐5‐methoxy‐2‐oxo‐1,3,4‐thiadiazol‐3‐ylmethyl)‐L‐cysteine (VI). The chloroform‐soluble metabolites were identified as the oxygen analogue of methidathion [S‐(2,3‐dihydro‐5‐methoxy‐2‐oxo‐1,3,4‐thiadiazol‐3‐ylmethyl)O, O‐dimethyl phosphorothioate] (V), the sulphoxideII, and the hydroxy compound 2,3‐dihydro‐3‐hydroxymethyl‐5‐methoxy‐1,3,4‐thiadiazol‐2‐one. The oxygen analogue of methidathion (V) was found in small amounts, corresponding to<5% of the added methidathion. Demethyl‐methidathion (IV) appeared