The H3B.NH3 at 200 K is orthorhombic, space group Pmn21, with a 5.421(5), b 4.945(4), and c 5.023(6) .ANG.; Z = 2. The structure was solved by Patterson methods and refined by least-squares to R = 0.0926. The B-N bond length is 1.564(6) .ANG.. The B-N length was calcd. by ab initio IGLO methods. Solvent effects appear to account for the obsd. differences.