The structure of nitranilic acid hexahydrate has been determined by use of partial three-dimensional data. The cell dimensions are: a= 3-657, b= 19.399, c= 9"184/~, B=94.28 °. The space group is P21/c and Z= 2. The geometry of the carbon ring system is similar to that found in chloranilate and nitranilate ions, and the carbon-oxygen bonds are of equal length (1-22 A, the same as in nitranilate). There is thus strong evidence that the substance is hydrated hydronium nitranilate. The ring is planar, but the nitro group is out of the ring plane.