Ultrastructual investigations and chemical determinations of compression wood from Abies balsamea, Picea mariana and Pinus strobus after decay by white or brown rot fungi demonstrated that this type of wood is more resistant to decay than normal wood. Hyphae colonizing compression wood were found in cell lumina and intercellular spaces whereas normal wood cell had hyphae only in cell lumina. Compression wood did not alter the type of cell wall degradation produced by the various fungi tested, but the rate and extent of decay were limited. The white rot fungus, Trametes versicolor, caused a nonselective attack of all cell wall components as indicated by erosion of secondary wall layers and middle lamellae. A selective removal of lignin occurred throughout the cell walls of wood decayed by the other white rot fungi (Phellinus pini, Phlebia tremellosa and Scytinostroma galatinum) that were evaluated. The brown rot fungi, Fomitopsis pinicola and Oligoporus placentus caused a diffuse removal of polysaccharides from both compression wood and normal wood. Tension wood from Populus temuloides and Acer rubrum decayed by white or brown rot fungi had similar amounts of decay to those observed in normal wood. Ultrastructual observations, however, showed striking differences in the progressive stages of decay. Hyphae of Tremetes versicolor located in cell lumina did not cause erosion or severe degradation of the adjacent, underlying gelatinous layer associated with tension wood. A typical nonselective degradation of the secondary wall layers and middle lamellae, however, occurred beneath this gelatinous layer. In areas of advanced degradation, the secondary walls and middle lamellae between cell walls were completely degraded leaving only the gelatinous layer and cell corner regions of the middle lamellae. Brown rot fungi were able to degrade the gelatinous layer and other cell wall layers resulting in extensive degradation of polysaccharides.