Herbivores have developed a wide array of countermeasures to overcome plants' chemical defences. Larvae of the cabbage white butterfly, Pieris rapae, feed exclusively on plants of the Brassicales order, which are defended by the glucosinolate-myrosinase system. The defensive function of this system comes from toxic isothiocyanates that are formed when glucosinolates are hydrolysed by myrosinases upon tissue damage. Here we show that P. rapae larvae convert benzylglucosinolate to phenylacetylglycine, which is released in their faeces. Feeding experiments with isotopic tracers suggest that phenylacetonitrile and phenylacetic acid are intermediates in this conversion. We also identified additional glycine and isoserine (2-hydroxy-3-aminopropanoic acid) conjugates with benzoate and indole-3-carboxylate from P. rapae faeces extracts. This is the first description of such conjugates from lepidopteran insects.