We have determined the crystal structure of ammonia monohydrate phase II (AMH II) employing a combination of ab initio computational structure prediction and structure solution from neutron powder diffraction data using direct space methods. Neutron powder diffraction data were collected from perdeuterated AMH II using the D2B high-resolution diffractometer at the Institut Laue-Langevin. AMH II crystallizes in space-group Pbca with 16 formula units in a unit-cell of dimensions a = 18.8285(4) angstrom, b = 6.9415(2) angstrom, c = 6.8449(2) angstrom, and V = 894.61(3) angstrom(3) [rho(deuterated)(calc) = 1187.56(4) kg m(-3)] at 502 MPa, 180 K. The structure is characterized by sheets of tessellated pentagons formed by orientationally ordered O-D center dot center dot center dot O, O-D center dot center dot center dot N, and N-D center dot center dot center dot O hydrogen-bonds; these sheets are stacked along the a-axis and connected by N-D center dot center dot center dot O hydrogen bonds alone. With the exception of the simple body-centered-cubic high-pressure phases of ammonia monohydrate and ammonia dihydrate, this is the first complex molecular structure of any of the high-pressure stoichiometric ammonia hydrates to be determined. The powder structure solution is complemented by an ab initio structure prediction using density functional theory which gives an almost identical hydrogen bonding network.