Cyanogenic compounds occur widely in the plant kingdom. Therefore, many herbivores are adapted to the presence of these compounds in their diet by either avoiding cyanide release or by efficient cyanide detoxification mechanisms. The mechanisms of adaptation are not fully understood. Larvae of Pieris rapae (Lepidoptera: Pieridae) are specialist herbivores on glucosinolate-containing plants. They are exposed to cyanide during metabolism of phenylacetonitrile, a product of benzylglucosinolate breakdown catalyzed by plant myrosinases and larval nitrile-specifier protein (NSP) in the gut. Cyanide is metabolized to β-cyanoalanine and thiocyanate in the larvae. Here, we demonstrate that larvae of P. rapae possess β-cyanoalanine activity in their gut. We have identified three gut-expressed cDNAs designated PrBSAS1-PrBSAS3 which encode proteins with similarity to β-substituted alanine synthases (BSAS). Characterization of recombinant PrBSAS1-PrBSAS3 shows that they possess β-cyanoalanine activity. In phylogenetic trees, PrBSAS1-PrBSAS3, the first characterized insect BSAS, group together with a characterized mite β-cyanoalanine synthase and bacterial enzymes indicating a similar evolutionary history.