AbstractMicrotubules, 210–230 Å in diameter and many microns in length, with an associated “clear zone” around them, 450–550 Å in diameter, are demonstrated in large numbers in cells of theStratum Granulosumof newborn rat epidermis. Few microtubules are found in the uppermost portion of theStratum Granulosumnor are they found above this layer in theStratum Corneum. Few microtubules are found in the germinal cells of the epidermis. Upon entering theStratum Granulosum, cells undergo shape changes involving elongation and flattening. Microtubules have elsewhere been implicated in whole cell movement and in the movement of portions of cytoplasm, including specifically cell elongation and the related functions of maintenance of asymmetric cell shapes and projections. Evidence is presented here in support of the hypothesis that microtubules are the basic cytoplasmic structure responsible for this motility. Two cortical arrays of microtubules, parallel and orthogonal in orientation, are demonstrated at different stages of cell flattening in epidermis. If the mode of action of microtubules is undulation and resulting “intracytoplasmic peristalis,” these orthogonal arrays represent a mechanism for the formation of the shape of squamous cells. The similar orientations of microtubule populations in many cells would seem to indicate that microtubule undulation and the resulting interaction between neighboring cells accounts for the transmission of axes of orientation