In the tunicate, Polyandrocarpa misakiensis, transdifferentiation occurs in the multipotent atrial epithelium during morphallactic bud development. Irradiation (10–80 Gy) or aphidicolin (10 μg/mL) blocked this process severely, although the atrial epithelium could form organ placodes. The placodes consisted of cuboidal cells with a high nucleus : cytoplasm ratio and were lacking the alkaline phosphatase antigen from the cell surface, suggesting that the atrial epithelium might undergo dedifferentiation without initiating cell cycling. Irradiated buds could resume organogenesis in temporal accordance with the restoration of mitotic activity. Bud pieces irradiated at 40 Gy were juxtaposed with unirradiated counterparts. In the operated buds, irradiated, non-dividing cells participated in organogenesis at the site of juxtaposition in cooperation with the unirradiated, dividing cells. These results have shown that in P. misakiensis the cell division cycle, probably DNA replication, is indispensable for transdifferentiation of the atrial epithelium, although every cell in the organ rudiment need not enter cell cycling. We suggest that homoiogenetic induction occurs between dividing cells and non-dividing cells.