The distribution of lignin in opposite wood has been studied by removing the polysaccharides with hydrofluoric acid and examining the resulting lignin skeletons in the electron microscope. The thick S3 layer was more highly lignified than the S1 and S2 layers in Abies balsamea, Picea rubens, Pinus resinosa, and Tsuga canadensis. In Picea rubens, but not in the other species, there was, adjacent to the S3 layer, a transition zone in S2 with a high concentration of lignin. The S3 layer varied considerably in thickness and was often buckled, especially in the latewood. The structure of the bordered pits was that observed in the original wood. The margo, the torus, and the initial pit border were all highly lignified.