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We have characterized ciliogenesis in a mutant Tetrahymena thermophila that both fails to regain motility following deciliation and that fails to complete cytokinesis. Scanning electron microscopic (SEM) observations revealed that starved deciliated cells regenerated fewer, shorter cilia at the restrictive temperature than similarly treated cells incubated at the permissive temperature. Transmission electron microscopic evaluation of isolated, regenerated cilia revealed no structural abnormalities. Incorporation of S-35 methionine was similar during ciliary regeneration at both the restrictive and permissive temperatures, indicating the mutant phenotype was not due to a simple failure in translation or transcription. Mutant cells incubated in growth medium at the restrictive temperature arrested in cytokinesis and assembled a large number of abnormally short cilia. These cells also developed irregular surface projections that were not visible on wild-type cells. These observations suggest that ciliogenesis can be initiated in growing cells as well as in starved deciliated cells but that elongation is inhibited before cilia reach full length. The mutation was named dcc for defective in ciliogenesis and cytokinesis.