The structure and internal dynamics of methyl beta-C-lactoside in aqueous solution has been derived by H-1 NMR and molecular modeling. A simulated annealing procedure under nuclear Overhauser effect (nOe) restraints produced a family of 50 structures. However, no single one of them was satisfactory; for each a rather important deviation between experimental and simulated data was found. Examination of the H-H correlation times given by off-resonance ROESY and comparison of the experimental and simulated coupling constants related to the interglycosidic dihedral angles indicated that the problem arose from internal mobility around the psi angle. Using the result of a phi/psi map, we have tried to find the best agreement between experimental and back-calculated nOe distances with two families having the same angle phi but different psi values. The best fit is obtained for 40% of (phi(A) = 295 degrees; psi(A) = 300 degrees) and 60% of (phi(B) = 295 degrees; psi(B) = 130 degrees). This result represents the first step toward comparison of the structure and dynamics of C- and O-glycosides in aqueous solution.