The sheath of Sphaerotilus natans is composed of cysteine-rich peptide and polysaccharide moieties. The polysaccharide was prepared by treating the sheath with hydrazine, and was determined to be a mucopolysaccharide containing beta-D-GlcA, beta-D-Glc, alpha-D-GalN, and beta-D-GalN. To elucidate the structure of the peptide, the sheath was labeled with a thiol-selective fluorogenic reagent, 4-(aminosulfonyl)-7-fluoro-2,1,3-benzoxadiazole. Enantiomeric determination of the S-derivatized Cys in the fluorescent sheath suggested that it contained L-Cys mainly. Fluorescent cysteinylglycine was detected in the partial acid hydrolysate of the fluorescent sheath. The sheath-degrading enzyme secreted by Paenibacillus koleovorans produced a fluorescent disaccharide-dipeptide composed of GalN, Gly, and N-acetylated Cys from the fluorescent sheath. The disaccharide and dipeptide moieties were found to be connected by an amide bond. Based on these results, the sheath was deduced to be formed by association of a mucopolysaccharide modified with N-acetyl-L-cysteinylglycine.