Microwave spectra of H314N-H14NO3, H315N-H14NO3, H314N-H15NO3, and H314N-D14NO3 have been obtained by Fourier transform spectroscopy in a supersonic jet. The spectra are consistent with a hydrogen- bonded structure having a planar arrangement of the heavy atom frame and a linear or near-linear hydrogen bond involving the HNO3 proton and the lone pair on the ammonia. The hydrogen bond length obtained from the rotational constants is 1.736(63) Å and the nitrogen-nitrogen separation is 3.344(12) Å. The NO single bond of the nitric acid and the C3 axis of the NH3 form angles of 54(4)° and 21(3)°, respectively, with the line joining the centers of mass of the monomers. The hydrogen bond length is significantly shorter than that observed in related complexes such as H3N-HCI and H3N-HBr, indicating a particularly strong interaction between ammonia and nitric acid.