Manganese containing superoxide dismutase from E. coli has been investigated through paramagnetic NMR spectroscopy. The spectrum of the native form was rationalized using a tau(s) = 3x10(-11) s for the Mn(III) ion, consistent with previous estimates from NMRD measurements. Mn(III) has been replaced by a Co(II) ion and a tentative assignment of the NMR spectrum of the Co(II)-substituted derivative has been proposed, based on T1, chemical shifts and 1D-NOE data. The metal coordination geometry is provided by three histidines and a carboxylate group. The presence of a solvent molecule as a loosely bound fifth ligand is also proposed. The NMR data of the Co(II)-substituted derivative of E. coli MnSOD differs from those of Co(II)SOD from other bacterial sources. This suggests that Co(II) substitution is an efficient method to address the problem of metal ion selectivity in superoxide dismutase.