The microtubule is a long hollow cylinder, 23 nm outside diameter and of indeterminate length, often many micrometers long. The wall of the microtubule, a curved sheet of 5 nm thickness, is seen to best advantage where it is split open and flattened against the carbon film as a non-overlapping, single sheet (fig. 1). Optical diffraction and image reconstruction techniques show that the wall is composed of subunits arranged on a 4 x 5 nm lattice (fig. 2). Each of these 4 x 5 nm subunits corresponds to a 55,000 MW protein subunit characterized biochemically and called tubulin.It is well established that microtubules are composed of two similar but distinct types of tubulin, called alpha and beta. In most negatively stained specimens, however, the 4 x 5 nm subunits are identical in structure. A small structural difference between alternate 4 x 5 nm subunits is seen under certain conditions (Amos and Klug, 1974; Baker and Amos, 1978), but in the present discussion we will ignore the distinction between alpha and beta subunits and pretend that the microtubule is composed of a single type of subunit.