The vegetative role of the germinal micronucleus of Tetrahymena thermophila was studied. Amicronucleate cells that had spontaneously arisen in star strains (very aged clones with defective micronuclei) were observed in B^* as well as in A^* and in C^* and were found to lack the oral apparatus and to have disordered ciliary rows. To reduce uncertainty given the very small micronucleus and possible effects of aging factors, we induced amicronucleate cells in a young clone by treatment with the antitubulin drug, nocodazole, and observed their cortical structure and nuclei. Amicronucleate cells gradually lost their oral apparatus and then their ciliary rows became disordered, even without cell division, and they became crinkled cells. It can be concluded that the vegetative micronucleus appears to play a critical role in the maintenance of the cortical structure, especially of the oral apparatus.