A difference in the distribution of microtubules in the peripheral cytoplasm of developing fibres has been observed between normal apple wood and that infected with ‘rubbery wood’ disease. In normal wood, where microtubules are abundant, the fibre wall is lignified, compact in texture and of limited thickness. In rubbery wood, where there are fewer microtubules, lignification is not complete in the fibre wall, which has a coarse, loose texture, and is abnormally thick. The whole stem is highly flexible. The pit membrane, which is unlignified in both normal and rubbery wood, has no microtubules adjacent to the plasmalemma. It is suggested that microtubules are concerned in channelling into the wall substances, such as lignin precursors, which are essential for certain steps in the polymerization of the amorphous component of the wall.