Aminopeptidase P (AMPP) from Escherichia coli cleaves the N-terminal residue from an oligopeptide if the second residue is proline. The active site contains a dinuclear metal centre. Following earlier structural analyses of crystals in space groups P6(4)22 and I4(1)22, the structure of AMPP has been solved and refined in the orthorhombic space group C222(1) at 2.4 A resolution. There are six subunits in the asymmetric unit. These are arranged in two types of tetramer. One tetramer comprises four crystallographically independent subunits, while the other comprises two pairs of subunits related by a crystallographic twofold axis. The final model of 20 994 protein atoms, 1618 water molecules and 12 metal atoms refined to residuals R = 0.195 and R(free) = 0.215. The molecular structure confirms most of the previously reported features, including the subunit-subunit interfaces in the tetramer and persistent disorder at some residues. The metal-ligand bond lengths at the active site suggest that one of the two Mn atoms is five-coordinate rather than six-coordinate.