The structure of bovine pancreatic trypsin inhibitor (BPTI) has been solved at 2.1 A resolution in a new crystal form (space group P6422 with unit-cell dimensions a = b = 95.0, c = 158.1 A). The asymmetric unit is a pentamer, but a decamer is created by application of crystallographic symmetry. The decamer of BPTI is only the fourth such assembly reported to date in the Protein Data Bank.