The process of skeletal micritization by boring thallophytes (algae and fungi) is known from modern carbonate environments and probably occurred as far back as the Ordovician. Boring thallophytes, probably fungi, possibly algae, were the cause of at least some skeletal micritization in Devonian reef complexes in western Canada. In situ endolithic filaments, of 3 μm and 5 μm diameter, occur associated with a micrite rim and micrite tubules in the corallite of an Upper Devonian tetracoral from the Miette reef complex. The filaments are found in the micrite rim around the coral, in the micrite tubules, and in unmicritized parts of the corallite, often oriented normal to the corallite wall and with the same trend as the tubules. Infestation and micritization began well before the corallite was buried, probably while the coral was alive, and continued for some time after death.