Abstract: A rotary drill bit, for drilling holes in subsurface formations, comprises a bit body, cutting structures mounted on the bit body, and a fluid supply system for supplying drilling fluid to the surface of the bit body, to cool and clean the cutting structures. The fluid supply system comprises a number of nozzles mounted in the bit body, a main passage in the bit body and a number of auxiliary passages leading from the main passage to the nozzles. Some or all of the passages are at least partly lined with an erosion-resistant lining material. The lining material may comprise a hard facing applied to the internal surface of a preformed passage or a rigid tube which itself defines the internal surface of the passage.