Longitudinally aligned microtubules, about 220 A in diameter, have been seen in the protoplasmic cylinders of the following spirochetes (symbiotic in the hindguts of dry-wood and subterranean termites): Pillotina sp., Diplocalyx sp., Hollandina sp. They are also present in a gliding bacterium from Pterotermes occidentis. These microtubules are probably composed of tubulin, as determined by staining with fluorescent antibodies to tubulin and comigration with authentic tubulin on acrylamide gels. Treponema reiteri lack tubulin by these same criteria. These observations support the hypothesis of the symbiotic origin of cilia and flagella from certain spirochetes.