Cucumisin is a diisopropyl fluorophosphate-sensitive enzyme. The inactivation by DFP is accompanied by the formation of 1 mol of labeled serine residue per mol of enzyme. From the soluble portion of the chymotryptic digest of the diisopropyl phosphoryl derivative of cucumisin, two peptides containing phosphorus were isolated; their amino acid sequences were determined to be Gly-Thr-Ser(P)-Met and Asn-Ile-Ile-Ser-Gly-Thr-Ser(P)-Met, respectively. The four residues Gly-Thr-Ser-Met in the above amino acid sequence are identical with those of subtilisin.