Most carbonate buildups of Dinantian age are mud-mounds lacking direct evidence of abundant framework organisms. This contribution describes apparently unique structures containing abundant framebuilding organisms interpreted as true reefs. They occur in the Red Hill Oolite, part of the Carboniferous Limestone succession in the Furness area of northwest England. Reefs were initiated by the attachment of numerous Syringopora colonies to a firm substrate. Encrusting organisms, dominantly the supposed foraminifcr Aphralysia, colonised sediment and corallite surfaces leading to the development of a rigid framework. Thrombolites also assisted in the establishment of bindstone textures. During the later stages of reef growth, Syringopora became less common and its place in the reef was taken by upright, branching growths of solenoporoid algae. Rapid sedimentation and subsidence resulted in reefs with near vertical sides, but little topographic expression on the sea-floor during growth. The occurrence of these reefs cannot be attributed to any single environmental factor but probably resulted from an unusual combination of favourable circumstances. D Calcareous algae, Carboniferous, corals, Dinantian, foraminifera, reefs, thrombolites.