The cytological changes accompanying the development of red blood cells of trout were studied. These changes appear to parallel closely those seen in the maturation of red blood cells of mammals. Immature erythrocytes of the trout contain mitochondria, Golgi complex, rough and smooth endoplasmic reticulum, centrioles, bundles of microtubules and numerous ribosomes in their cytoplasm. With progressive differentiation and maturation such cells appear to shrink in size, acquire a biconvex ellipsoid form, lose most of their cytoplasmic organelles and concomitantly increase their hemoglobin content. Mitochondrial degradation begins early in the process of maturation and usually consists of the following sequence of morphological changes within them: the appearance of small dense bodies, degeneration of the cristae and the formation of lamellar bodies. Degenerating mitochondria are often observed bulging from the surface of the cell, suggesting that they are in the process of being extruded from it. However, this observation does not preclude the possibility that mitochondria may also be disposed of in situ through the action of lysosomal enzymes. It is suggested that the disappearance of mitochondria and other cytoplasmic organelles contributes to the volumetric shrinkage which accompanies the maturation of trout red blood cells. The existence of an equatorially oriented “marginal band” of microtubules has been confirmed in this material in agreement with studies on other species. A band of microtubules was also observed in trout thrombocytes. It is concluded that the marginal band plays an important role in the maintenance of the flat ellipsoidal shape of these cells as has been suggested by other investigators studying other vertebrate forms.