The positions of the five crystallographically independent hydrogen atoms in the mineral brushite, CaHPO4,2H2O, which crystallizes with Z= 4 in the non-centrosymmetric monoclinic space group la(Cs4), have been established by neutron-diffraction analysis based on 632 independent reflexions from synthetic crystals. Anisotropic three-dimensional least-squares refinement of all the atoms in the structure to R 0·033 gives final estimated standard deviations of 0·002 for calcium ions, ca. 0·006 for phosphorus and oxygen atoms, and 0·010–0·015 Å for hydrogen atoms. The final Fourier difference synthesis shows no significant spurious peaks and there is no evidence that any hydrogen atom occupies several sites; the anionic hydrogen is 1·00(1)Å from O(1), which has the longest [1·608(6)Å] P–O bond. The two water molecules co-ordinate somewhat differently; both are essentially coplanar with the oxygen atoms to which they are hydrogen bonded, but water (2) has an unusually long bond of 3·09 Å to water (1). The other four distinct hydrogen bonds in the structure, which closely resembles that of the arsenic analogue, pharmacolite, have O ⋯ O lengths ranging from 2·68 to 2·83 Å.