The structure and molecular weight of the microtubule-dependent ATPase, dynein, was first established by scanning transmission electron microscopy (STEM) of dynein isolated from the cilia of Tetrahymena. It was shown that dynein consists of three globular heads joined to a common base by three slender, flexible strands. The globular heads have a diameter of 10-12 nm and the strands are 24 nm in length, such that the particles are 35 nm overall. Mass analysis by integration of electron scattering intensities in the STEM established a molecular weight of 1.9 million, with each head contributing 420,000. Several lines of evidence suggested that the base anchors the dynein to the A-tubule and the three independent, globular heads interact with the B-tubule of the adjacent outerdoublet in an ATP-dependent reaction to produce a force for sliding.