Analysis of x-ray diagrams of oriented hydrated cytoplasmic microtubules shows that the tubule wall extends from about 70 to 150 A radially. The central region of the wall appears homogeneous, but the outside surface is subdivided by vertical grooves separating the 13 protofilaments and by a steep 10-fold family of grooves. The inside surface is dominated by the 10-start grooves with no clear subdivision between the protofilaments.