Negatively stained arrays of flagellar doublet microtubules have been tilted about an axis perpendicular to the tubule axes and imaged at angles of ±20°. The images have been filtered to enhance the 4-nm periodicities in the tubulin lattice, and the resulting filtered images interpreted by comparison with simulated projections of tilted helical lattices. The different appearances of the edges of filtered doublet tubules for positive and negative tilt angles suggest that the 4-nm lattices of both subfibres are of the same hand, with the 3-start helices in each being left-handed. The information has been incorporated in a model proposed for the arrangement of subunits in doublet microtubules, which is described in the preceding paper.

*Present address: Department of Anatomy, Harvard Medical School, Boston, Massachusett 02115, U.S.A.