The immunoglobulin binding protein, segment B1 of protein G, has been studied experimentally as a paradigm for protein folding. This protein consists of 56 residues, includes both beta sheet and alpha helix and contains neither disulfide bonds nor proline residues. We report an all-atom molecular dynamics study of the native manifold of the protein in explicit solvent. A 2-ns simulation starting from the nuclear magnetic resonance (NMR) structure and a 1-ns control simulation starting from the x-ray structure were performed. The difference between average structures calculated over the equilibrium portion of trajectories is smaller than the difference between their starting conformations. These simulation averages are structurally similar to the x-ray structure and differ in systematic ways from the NMR-determined structure. Partitioning of the fluctuations into fast ( 20 ps) components indicates that the beta sheet displays greater long-time mobility than does the alpha helix. Clore and Gronenborn [J. Mol. Biol. 223:853-856, 1992] detected two long-residence water molecules by NMR in a solution structure of segment B1 of protein G. Both molecules were found in the fully exposed regions and were proposed to be stabilized by bifurcated hydrogen bonds to the protein backbone. One of these long-residence water molecules, found near an exposed loop region, is identified in both of our simulations, and is seen to be involved in the formation of a stable water-mediated hydrogen bond bridge. The second water molecule, located near the middle of the alpha helix, is not seen with an exceptional residence time in either as a result of the conformation being closer to the x-ray structure in this region of the protein.