A series of thiomercuric derivatives of mono-and disaccharides 1-7, in which methylmercury or phenylmercury is covalently attached to anomeric thioglycosides, were synthesized for structure-function studies of glycosidases. Thiomethylmercuryl xylobiosides 5 and 6 were found to inhibit intracellular xylanase-T6 in a competitive manner, with Ki values of 0.35 mM and 0.01 mM, respectively. These inhibitors have been co-crystallized with the enzyme and are being used for X-ray analysis. 1-(Thiomethylmercuric)-β-D-xyloside (3) affords crystals belonging to the orthorhombic space group P212121 and at 293(2) K: a = 6.7510(2), b = 9.7140(2), c = 29.4770(9) Å, V = 1933.08(9) Å, Z = 8, R(F2) = 0.0329, and R (F2) = 0.0626. There are two molecules (A and B) in the asymmetric unit, and each one shows an almost linear S-Hg-C arrangement. Biological tests on 1-7 indicated that they exhibit potent fungicidal and herbicidal activities.