A large concretion in the black shale above Springfield Coal Member (No. V), Desmoinesian Series, Pennsylvanian, from the Blackfoot #5 mine, Pike County, Indiana, contains a piece of fossil wood with excellent preservation of cell structure. Both ends of the wood extending outward to the extremities of the ovoid concretion are replaced by a black calcareous rock in which there is no fossil wood structure. These end zones have sharp contacts with the concretion matrix and their dimensions correspond to those of the fossil wood plus associated calcite veins. These relations indicate that the concretion must have been well-established before the wood had suffered any structural disintegration through bacterial activity. The preservation of the central section of the wood was the result of early concretion formation, while the ends continued to rot away as they were open to the environment. -- This is further evidence for the rapid early diagenetic origin of many concretions, particularlly those associated with well-preserved fossils.