Abstract: This paper is a study of the fine structure of peripheral blood cells in four species of fresh-water fish with electron microscopy and light microscopy.The erythrocytes are oval or ellipsoid in shape. The nucleus contains electron-dense clumps of chromatin and a small nucleolus. The cytoplasm is filled with finely granular matrix. The rod-like mitochondria contain longitudinally oriented tube-shaped cristae. Under the plasmolemma, bundles of microtubules can be seen.The lymphocytes are spherical cells, with an intensely staining round or slightly indented nucleus and a thin rim of cytoplasm. Some slender pseudopods project from the cells. The cytoplasm contains abundant ribosomes, rough endoplasmic reticulum and several mitochondria. The Golgi apparatus is located opposite the indentation of the nucleus.The thrombocytes occur in aggregates and are round or oval in shape. The nucleus, which occupies most of the cell, contains densely clumped chromatin. The cytoplasmic matrix is electrondense and various vacuoles occur in the ectoplasm.The monocytes are easily identified by their kidney-shaped nuclei and several vacuoles located in extensive cytoplasm.All the granulocytes contain only few organelles and an eccentric nucleus of various forms. The neutrophil contains two types of specific granules. Some are spherical and contain electron-dense material among the limpid matrix. Others are hollow cylinders which contain 1—3 parallel microtubules in their transparent matrix. The eosinophilic granules are characterized by the moderately electron-dense matrix enclosing a pleomorphie crystalloid body. The basophilic granules are large, oval or spherical, with electron-dense matrix.Besides the description of the fine structure of the blood cells, we also compare the morphology of the blood cells of fresh-water fish with those of other vertebrates.