Neuronal microtubules have been reassembled from brain tissue homogenates and purified. In reassembly from purified preparations, one of the first structures formed was a flat sheet, consisting of up to 13 longitudinal filaments, which was identified as an incomplete microtubule wall. Electron micrographs of these flat sheets and intact microtubules were analyzed by optical diffraction, and the surface lattice on which the subunits are arranged was determined to be a 13 filament, 3-start helix. A similar, and probably identical, lattice was found for outer-doublet microtubules. Finally, a 2-D image of the structure and arrangement of the microtubule subunits was obtained by processing selected images with a computer filtering and averaging system. The 40 x 50 A morphological subunit, which has previously been seen only as a globular particle and identified as the 55,000-dalton tubulin monomer, is seen in this higher resolution reconstructed image to be elongated, and split symmetrically by a longitudinal cleft into two lobes.