The effects of the antitumor drug taxol on the microtubular axonemes of the heliozoon Actinophrys sol have been investigated. The drug induces polymerization of microtubules as shown by a large increase in the length and number of microtubular arrays. The interaction between microtubules and microtubule-associated proteins is also affected, with the result that the normal geometric patterning within the microtubular arrays is disturbed. This is due to the loss or inactivation of long intermicrotubule links. As a result, arms lose their rigidity. Because the drug stabilizes polymerized microtubules, C-shaped profiles and other signs of poor microtubule preservation are absent in taxol-treated cells.