Methylmercury- d, l-selenocysteinate monohydrate complex, a key model for the methylmercury(II)-selenoprotein interaction in vivo, has been prepared via the reaction of seleno- d, l-cysteine (from prior reduction of seleno-D,L-cystine) with methylmercury(II) hydroxide and characterized by single-crystal X-ray diffractometry. Crystals are orthorhombic, space group P2 1 2 12 1, with unit cell dimensions a = 6.384(1), b = 26.309(4), c = 5. 292(1) Å ,and Z = 4. The structure was solved by Patterson and Fourier techniques and refined by least-squares methods to a final R of 0.064 for 762 independent observed reflections measured on an automatic diffractometer. The selenoamino acid is coordinated to Hg via a deprotonated selenohydryl group [Hg-Se 2.469(4) Å]. A very weak intramolecular Hg…O interaction [2.93(2) Å] to a carboxylate group, and a similarly weak intermolecular Hg…Se interaction [3.737(4) Å] to the adjacent selenocysteine moiety, are present. Individual molecules of the complex are linked by hydrogen bonds to solvent water molecules. Infrared and Raman spectra are reported, 1H and 199Hg nuclear magnetic resonance data, together with the structural data, support the view that the selenocysteinate group interacts more strongly with CH 3Hg + than the sulfur analogue. Strong Hg-Se bonding to selenocysteinate residues may play an important role in selenium protection.