Interactions between hydrophobic groups in water, as well as biomolecular hydration more generally, 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 (2M(S)-6M(L) x 46H2O), cubic structure II (16M(S) x 8M(L)-136H2O) and hexagonal structure H (M(L) x 3M(S) x 2M(S) x 34H2O) 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 x 30.33H2O. 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.