Ascidian early embryonic cells undergo cell differentiation without cell-cleavage, thus enabling to mix cell-fate determinants in single cells, which will not be possible in mammalian systems. Either cell in a 2-cell embryo (2C cell) has multiple fates and develops into any cell-types in a tadpole. To find the condition for controlled induction of a specific cell-type, cleavage-arrested cell-triplets were prepared in various combinations. They were 2C cells in contact with a pair of anterior neuroectoderm cells from 8-cell embryos (2C-aa triplet), with a pair of presumptive notochordal neural cells (2C-AA triplet), with a pair of presumptive posterior epidermal cells (2C-bb triplet) and with a pair of presumptive muscle cells (2C-BB triplet). The fate of the 2C cell was electrophysiologically identified. When 2-cell embryos had been fertilized 3 hours later than 8-cell embryos and triplets were formed, the 2C cells became either anterior-neuronal, posterior-neuronal or muscle cells, depending on the cell-type of contacting cell pair. When 2-cell embryos had been fertilized earlier than 8-cell embryos, most 2C cells became epidermal. When 2- and 8-cell embryos had been simultaneously fertilized, the 2C cells became any one of three cell-types described above or the epidermal cell-type. Differentiation of the ascidian 2C cell into major cell-types was reproducibly induced by selecting the type of contacting cell-pair and the developmental time difference between the contacting cell-pair and 2C cell. We discuss similarities between cleavage-arrested 2C cells and vertebrate embryonic stem cells and propose the ascidian 2C cell as a simple model for toti-potent stem cells.