Sea-urchin sperm tails (Strongylocentrotus purpuratus) were obtained by amputation in synthetic sea water and were purified by differential centrifugation. Most of the arms of the outer nine doublets and soluble matrix proteins were removed by this treatment. The central pairs of microtubules were dissolved by dialysis against EDTA at pH 7.5. The extract contained essentially a single component, with a sedimentation constant of 6S, in amounts sufficient to account for the protein content of the central pairs. Incubation of the extract with colchicine-3H gave binding levels approaching 0.5–1.0 mole of colchicine per 105 g protein. Sucrose-gradient analysis showed that the bound-radioactivity profile coincided with the optical-density profile of the 6S protein. It is concluded that the 6S colchicine-binding protein is a subunit of microtubules.