The ultrastructural characteristics of fracture surfaces from southern pine and Douglas-fir specimens tested in tension at various angles to grain were examined. The fracture surface morphology was inspected using scanning electron microscopy. Three anatomical failure types were recognized: intercell failure, transwall failure, and intrawall failure. Certain failure characteristics were ascribed as a function of the magnitudes of parallel-and perpendicular-tograin tension and parallel-to-grain shear present in the specimen. In specimens tested in parallel-and perpendicular-to-grain tension, the thick-walled latewood cells were found to fail in a combination of transwall and intrawall failure. The intrawall failures were usually at the $1-$2 interface. The more thin-walled earlywood cells were more likely to exhibit abrupt, transwall failures. At intermediate angles of load to grain, surfaces indicative of the type found in pure shear tests were predominant. Perpendicular-to-grain tension failures resulted in mostly intercell failures. Ray cells consistently exhibited transwall failures. The failure surface frequently changed planes in all loading modes. This path transfer was inevidably associated with material discontinuities in the wood. When the path did transfer, all three failure types were observed. No significant species effect was observed. IntroductionBasic to the understanding of the mechanical behavior of a material is the appreciation of the mechanical behavior of its individual units and their interaction. For wood, this is not a simple matter. The relationship between the structural organization of wood and its mechanical behavior is extremely complex. Under external stress, the response of wood is a function of material interaction on several levels of complicated inhomogeneous structure. IOu (1964) summarized it as follows: