Monomeric sarcosine oxidase (MSOX) is a flavoprotein that catalyzes the oxidation of sarcosine to generate formaldehyde, glycine, and hydrogen peroxide, and is utilized in quantification of creatinine in serum. Crystal structure of MSOX from Bacillus sp. NS-129 (without ligand) has been determined at 1.86 Å. Unlike the published structures of MSOX from Bacillus sp. B-0618 (without or with carboxylate-containing ligand), the two molecules in the asymmetric unit adopt distinct conformations at the active site loop (Gly56 to Glu60) with a maximal root-mean-square (RMS) displacement of 3.3 Å for Cα atom of Arg59. The multiple conformations seen at the active-site loop suggest that high flexibility of the loop would be important for the activity of MSOX.