The crystal structure of pyridine is orthorhombic with space group Pna21, a = 1752.4(3), b = 896.9(2), and c = 1135.2(2) pm at 153 K and Z = 16. A low melting crystalline hydrate of pyridine was found by DTA to be a trihydrate. It is orthorhombic, too, with space group Pbca, a = 1244.4(3), b = 1783.2(6), and c = 679.1(2) pm at 223 K and Z = 8. Both structures were determined from single crystal MoKα diffractometer data and refined to unweighted R values of 0.044 for 2769 and of 0.074 for 932 observed independent reflections. Most apparent in the anhydrous structure is an arrangement of the molecular noncrystallogrpahic twofold axes in layers perpendicular to the c-axis direction and separated by c/4. With the ring centers not far from the points of a bcc lattice a ''molecular coordination number'' of 14 can be assigned to each of the four independent molecules. Their geometries are very similar and close to mm-C2v symmetry. In the structure of the trihydrate the water molecules are O-H ⋅ ⋅ ⋅ O hydrogen bonded in a new pattern of condensed four, five, and six-membered rings to form buckled layers, separated by b/2, which enclose the pyridine molecules. There is one hydrogen bond O-H ⋅ ⋅ ⋅ N from a water molecule to the pyridine molecule. All hydrogen atoms are ordered.