AbstractMany plant proteinase inhibitors have lysine at the P1 position, presumably to avoid hydrolysis by insect trypsins. Lepidopteran trypsins appear to have adapted to resist proteinase inhibitors through increased inhibitor hydrolysis and decreased binding to inhibitor hydrophilic reactive sites. Lepidopteran digestive trypsins prefer lysine at the P1 position and have substrate binding subsites more hydrophobic than trypsins from insects in other orders. All available sequences of sensitive and inhibitor‐insensitive insect trypsins were aligned with porcine trypsin, for which interactions with Kunitz and Bowman‐Birk inhibitor are known. After discounting conserved positions and positions not typical of sensitive or insensitive trypsins, the following residues were considered important to insect trypsin‐PI interactions (chymotrypsin numbering): 60, 94, 97, 98, 99, 188, 190, 213, 215, 217, 219, 228. These residues support the Neighbor Joining analysis tree branches separating sensitive and insensitive trypsin sequences. Primary sequences interacting with PIs are around the active site, with some forming part of the S1 (188, 217, 219 and 228) or S4 (99, 215) pockets. Arch. Insect Biochem. Physiol. 55:140–152, 2004. © 2004 Wiley‐Liss, Inc.