Planarians are well known for their remarkable regenerative capacity. This capacity to regenerate is thought to be due to the presence of totipotent somatic stem cells known as 'neoblasts', which have particular morphological characteristics. The totipotency of neoblasts was supported by Baguna's experiment, which involved the introduction of donor cells into irradiated hosts. However, since Baguna's experiment did not include the use of a phenotypic marker, the donor cells could not be traced. In the current study, a genetic mutant planarian, menashi, an eye-defective mutant that lacks the pigmented area in the eyes, was established. This planarian is excellent for tracing the fate of cells after their introduction into irradiated hosts. To investigate the differentiation potency more directly, a neoblast-rich fraction obtained from normal worms was transplanted into an X-ray-irradiated menashi strain. Planarians that survive X-ray irradiation were developed, and we observed the pigment of the area in the eyes of the regenerating planarians. This result suggests that the neoblast-rich fraction contains cells that can proliferate and differentiate. These cells can replace the cells and structures lost by X-ray irradiation and ablation, and they can also differentiate into eye pigment cells.