The degradation of wood by brown rot fungi has been studied intensely for many years in order to facilitate the preservation of in-service wood. In this work we used X-ray diffraction to examine changes in wood cellulose crystallinity caused by the brown rot fungi Gloeophyllum trabeum, Coniophora puteana, and two isolates of Serpula lacrymans. All fungi increased apparent percent crystallinity early in the decay process while decreasing total amounts of both crystalline and amorphous material. Data also showed an apparent decrease of approximately 0.05 Å in the average spacing of the crystal planes in all degraded samples after roughly 20% weight loss, as well as a decrease in the average observed relative peak width at 2¿ = 22.2°. These results may indicate a disruption of the outer most semi-crystalline cellulose chains comprising the wood microfibril. X-ray diffraction analysis of wood subjected to biological attack by fungi may provide insight into degradative processes and wood cellulose structure.