The stoichiometric complex formed between bovine β-trypsin and the Cucurbita maxima trypsin inhibitor I (CMTI-I) was crystallized and its X-ray crystal structure determined using Patterson search techniques. Its structure has been crystallographically refined to a final R value of 0.152 (6.0 - 2.0 Å). CMTI-I is of ellipsoidal shape; it lacks helices or β-sheets, but consists of turns and connecting short polypeptide stretches. The disulfide pairing is CYS-3I-20I, Cys-10I-22I and Cys-16I-28I. According to the polypeptide fold and disulfide connectivity its structure resembles that of the carboxypeptidase A inhibitor from potatoes. Thirteen of the 29 inhibitor residues are in direct contact with trypsin; most of them are in the primary binding segment Val-2I (P4) - Glu-9I (P4') which contains the reactive site bond Arg-5I - Ile-6I and is in a conformation observed also for other serine proteinase inhibitors.