Interactions between hydrophobic groups in water(1), as well as biomolecular hydration more generally(2-4), are intimately connected to the structure of liquid water around hydrophobic solutes. Such considerations have focused interest on clathrate hydrates: crystals in which a hydrogen-bonded network of water molecules encages hydrophobic guest molecules with which the water interacts only by non-directional van der Waals forces. Three structural families of clathrate hydrates have hitherto been recognized: cubic structure I (2MS . 6M(L). 46H(2)O) (ref, 5), cubic structure II (16M(S). 8M(L). 136H(2)O) (ref. 5) and hexagonal structure H (M-L. 3M(S). 2M(S). 34H(2)O) (refs 6, 7) hydrates (here M-L and M-S are the hydrophobic guest sites associated with large and small cavities, respectively). Here we report a new hydrate structure: 1.67 choline hydroxide;tetra-n-propylammonium fluoride . 30.33H(2)O. This structure has a number of unusual features; in particular the choline guest exhibits both hydrophobic and hydrophilic modes of hydration. Formally the structure consists of alternating stacks of structure H and structure II hydrates, and might conceivably be found in those settings (such as seafloor deposits over natural-gas fields) in which clathrate hydrates form naturally.