The sawfly species Athalia rosae (L.) (Hymenoptera: Tenthredinidae) is phytophagous on plants of the family Brassicaceae and thus needs to cope with the plant defence, the glucosinolate–myrosinase system. The larvae sequester glucosinolates in their haemolymph. We investigated how these compounds are metabolized by this specialist. When larvae were fed with ([14C]-labelled) benzylglucosinolate, one major degradation metabolite, with the same sum formula as benzylglucosinolate, was defecated. This metabolite was also found in the haemolymph along with desulfobenzylglucosinolate, which continuously increased in concentration. NMR spectroscopy in conjunction with LC-TOF-MS measurements revealed the major degradation metabolite to be desulfobenzylglucosinolate-3-sulfate, probably converted from desulfobenzylglucosinolate after sulfation at the sugar moiety. The enzymes responsible must be located in the haemolymph. Additionally, a putative sulfotransferase forms benzylglucosinolate sulfate in the gut from intact, non-sequestered glucosinolate. The corresponding desulfoglucosinolate sulfates were also detected in faeces after feeding experiments with phenylethylglucosinolate and prop-2-enylglucosinolate, which indicates a similar degradation mechanism for various glucosinolates in the larvae. This is the first report on glucosinolate metabolism of a glucosinolate-sequestering insect species.