Microtubules (MTs) are organized into distinct systems essential for cell shape, movement, intracellular transport, and division. Electron crystallographic analyses provide little information about how MTs produce diverse structures and functions, perhaps because they failed to visualize the last 10 residues of the alpha- and the last 18 of the beta-tubulin C-terminal tails (CTTs), which likely play a role in MT diversity. CTTs define conserved, nonallelic isotypes in mammals, are major sites of posttranslational modifications (PTMs), are binding sites for microtubule-associated proteins (MAPs), and determine MT motor processivity. Using mutagenesis and homologous gene replacement in Tetrahymena thermophila, we analyzed mutations, deletions, tail switches, and tail duplications of alpha- and beta-tubulin CTTs. We demonstrate that a tail is required for the essential function of both alpha- and beta-tubulin. However, the two tails are interchangeable, and cells grow normally with either an alpha or a beta tail on both tubulins. In addition, an alpha gene containing a duplicated alpha C terminus rescues a lethal mutant lacking all known posttranslational modification sites on the beta C terminus but cannot rescue deletion of the beta tail. Thus, tubulin tails have a second essential function that is not associated with posttranslational modification.