A quantitative scanning electron microscopic (SEM) study of the changes in microtubule orientations and arrays during secondary wall formation has been done on conifer (Abies sachalinensis Masters) tracheids. Microtubules have similar orientations as the microfibrils being deposited in the various wall layers. The density of microtubules is different in different stages of secondary cell wall formation. Microtubules are more closely arrayed in the tracheids forming the S2 than the S12 and S3. During S3 formation, sometimes 2-7 microtubules are closely arrayed, and form bundles about 80-350 nm wide. Bundles of microfibrils of similar width were also observed during S3 formation.