The NMR structure of the severe acute respiratory syndrome coronavirus (SARS-CoV) non-structural protein 7 (nsp7) in aqueous solution at pH 6.5 was determined and compared with the results of previous structure determinations of nsp7 in solution at pH 7.5 and in crystals of a hexadecameric nsp7/nsp8 complex obtained from a solution at pH 7.5. All three structures contain four helices as the only regular secondary structures, but there are differences in the lengths and sequence locations of the four helices, as well as between the tertiary folds. The present study includes data on conformational equilibria and intramolecular rate processes in nsp7 in solution at pH 6.5, which provides further insights into the polymorphisms implicated by comparison of the presently available three nsp7 structures.