Abstract. The fine structure and ATPase activity of the mitotic spindle in neoblasts of planaria were examined. In neoblasts, the cells have a large nucleus and nucleolus. Mitochondria are aggregated around the nucleus with chromatoid bodies adjacent. The cytoplasm contains little endoplasmic reticulum (ER) and few Golgi bodies but many free ribosomes, forming polysomes, can be seen throughout the cytoplasmic and spindle ground areas. In addition, centriolar bodies, atypical centrioles, can also be recognized in the cytoplasm. Ceils in the G 2 stage contain a pair of electron-dense bodies, both consisting of fibrogranules but differing from each other in fine structure and, in the mitotic stage, only one fibrogranular body can be recognized at each pole. ATPase activity was detected in the centriolar bodies in the G 2 and mitotic stages and in the ground area of the cytoplasm and spindle apparatus filled by free ribosomes. The activity associated with the microtubules differed with the developmental stage.