A neutron diffraction study of L-lysine monohydrochloride dihydrate, C6H15N202. C1.2H20, has been carried out. The structure is monoclinic, space group P2~ with Z = 2. Cell parameters are: a = 7492 (1), b = 13.320 (4), c= 5.879 (1)/~, fl= 97.79 (1) ° [Wright & Marsh, Acta Cryst. (1962), 15, 54]. Least-squares refinements based on 1240 reflections led to a conventional R value of 0.030. The neutron-diffraction results confirm the structure previously determined by X-ray diffraction [Raman Z. Kristallogr. (1959). 111, 301; Wright & Marsh (1962)]; the main new feature is the precise determination of the hydrogen atom positions. The refined thermal parameters were fitted to rigid-body motions; these calculations indicate axial librations of 8.8 (5) and 12.7 (4) ° for the a-NH + and ~-NH~ groups respectively. Assuming a threefold cosine potential, these librational amplitudes lead to estimated barriers to rotation of 7.0 kcal.mole -~ for the a-NH~, group and 2.9 kcal.mole -~ for the ff-NH3+ group.