Tetrahymena contains a micronucleus and a macronucleus. The micronucleus divides with typical mitosis, while the macronucleus divides amitotically. Although the mechanism responsible for macronuclear division was previously unknown, we clarified the organization of microtubules during macronuclear division. The macronuclear microtubules dynamically changed their distribution in an organized way throughout the macronuclear division. The macronuclear microtubules and the cytoplasmic microtubules cooperatively carried out the macronuclear division. When the micronuclear division was finished, p85 appeared at the presumptive division plane prior to the cytokinesis. The p85 directly interacted with calmodulin in a Ca2+-dependent manner, and p85 and CaM colocalized to the division furrow during cytokinesis. Moreover, the Ca2+/CaM inhibitor, W7, inhibited the direct interaction between p85 and CaM, the localization of both proteins to the division plane, and the formation of the division furrow. Thus, Ca2+/CaM and p85 have important roles in initiation and progression of cytokinesis in Tetrahymena.