Hydrazinium 3-amino-5-nitro-1,2,4-triazolide, N2H5+.C2H2N5O2, M(r) = 161.1, orthorhombic, Pbca, a = 5.392 (2), b = 13.316(2), c = 18.483 (6) angstrom, V = 1327.1 (7) angstrom3, Z = 8, D(x) 1.612 Mg m-3, lambda(Mo Kalpha) = 0.71069 angstrom, mu = 0.130 mm-1, F(000) = 672, room temperature, final R = 0.030 for 717 observed reflections [F > 5sigma(F)] out of 1157 independent reflections. The ring geometry is very similar to other substituted 1,2,4-triazoles and is planar within 0.003 angstrom. The structure is held together by an extensive network of hydrogen bonds.