Abstract The synthesis of selenium bound selenourea complexes of methylmercury, [CH3HgSeC(NH2)2]X (X=Cl, Br, NO3, ClO4) are described. A single crystal X-Ray analysis of the nitrate salt has provided for the first time bond length data pertinent to the biologically important mercury-selenium interaction. Crystals of [CH3HgSeC(NH2)2] NO3 belong to the space group PnZic with a = 7.524(1), b = 11.204(2), c = 9.738(2)A, and z = 4. The structure was solved and refined using 561 observed reflections measured on a Syntex P21 diffractometer to a final R value of 0.037. The mercury atom is linearly coordinated to the methyl group and the selenium atom of the selenourea with Hg-Se of 2.477(3)A. Strong Hg-Se bonding is indicated by the X-Ray data and by the 1H - 199Hg nmr coupling constants. A comparison of spectroscopic data for analogous thio and selenourea complexes is presented.