The paper is a study of the cytology of the regeneration cells (neoblasts) in Planaria vitta. The morphology of the living cells has first been examined to provide a reference for an investigation of the fixed neoblasts as studied by ordinary cytological, cytochemical and electron microscopical technics. A rather selective staining method has been devised based on the strong basophilic properties of the scanty cytoplasm. The morphology of the fixed neoblasts and their distribution in the intact animal have been described, using this method. The marked cytoplasmic basophilia was found to be exclusively due to ribonucleic acid, and not to desoxyribonucleic acid or acid mucopolysaccharides. The cytoplasm contains moderate to considerable amounts of basic proteins. Tyrosine, cysteine/cystin, arginine, lysine and perhaps histidine were present, while tryptophan could not be demonstrated. No enzymes could be demonstrated apart perhaps from cytochrome oxidase. The mitochondria are small and inconspicuous and more or less evenly distributed throughout the cytoplasm. A Golgi apparatus could not be demonstrated. The electron microscopic picture is very characteristic, because of the high electron density of the cytoplasm. This density is the result of the presence of a great number of ribonucleoprotein granules. Most of the granules are free and only a minor part bound to the membranes of the endoplasmatic reticulum. The interesting features of the cell membrane are discussed in relation to the structure of the parenchyma. The cytochemical properties of the neoblast (RNA and sulfhydryl-groupcontaining protein) and the fine structure as revealed in the electron microscope characterize the neoblast as a morphogenetically active cell.