AbstractStructure–activity relationship studies were conducted with early fourth‐instar larvae of a highly resistant strain of the diamondback moth,Plutella xylostella(Linnaeus) on (1) toxicity by topical appliction of 43 organotin compounds, and (2) the antifeedant effect of a selected number (17) of these compounds on treatedBrassica chinensis(Chinese cabbage) leaves.The toxicity data revealed that the triorganotins (R3SnX) were, without exception, more toxic than the commercial sample of malathion (84% active ingredient) used in the tests. Among the diorganotins, phenylcyclopentyltin oxide proved to be as active as malathion. Within the triorganotin series, the tricyclohexyltins were generally more toxic than the triphenyltins, the most active tricyclohexyltin compound being (c‐C6H11)3Sn(2‐pyridinethiolatoN‐oxide) (LC500.03 μg μl−1), which was almost 500‐fold more active than malathion. The most active compound in the triphenyltin class wasO, S‐bis(triphenyltin)mercaptoacetate (LC500.30 μg μl−1). Variations in the anionic X group resulted only in marginal changes in activity in the (c‐C6H11)3Sn series, but significant changes in activity were obtained with the Ph3Sn compounds, especially the ring‐substituted phenoxyacetates, (4‐ZC6H4)OCH2(O)COSnPh3. In the mixed triorganotin compounds an increase in activity was observed when one of the phenyl groups in Ph3SnOH was replaced by thep‐chlorophenyl group.In the antifeedant tests, the tricyclohexyltins were found to be generally more effective than the triphenyltins. In most cases, antifeedant activity paralleled the toxicity by topical application trends in the (c‐C6H11)3Sn series, but in the Ph3Sn series an inverse trend was observed. The diorganotin compound (c‐C5H9)PhSnO exerted a relatively pronounced antifeedant activity which was comparable with that of a number of triphenyltin derivatives.It was established from histological studies of the mid‐gut cross‐sections of the treated larvae that, in most cases, the organotins affected the columnar cells physiologically; an exception was noted for Ph3SnOC(O)C6H4COOH‐4 which, like malathion, caused severe mo