Cytochemical studies are used to identify fish leukocytes and as a basis for studying the functions of these cells in cellular immune responses. In this work, we investigated the morphological features and cytochemical properties of the blood cells in the armored catfish (Hoplosternum littorale), a South American teleost. Reticulocytes, which accounted for 8-24.6% of the red blood cell population, stained with brilliant cresyl blue and contained a granular material similar to residual RNA. Thrombocytes, lymphocytes, monocytes, neutrophils, heterophils and eosinophils were identified and characterized in blood smears stained with May Grünwald-Giemsa-Wright. The lymphocytes were small, round cells with a basophilic cytoplasm and contained no periodic acid-Schiff (PAS), peroxidase or non-specific esterase activity. The thrombocytes were usually fusiform, with a hyaline cytoplasm that was acidophilic when stained with alkaline toluidine blue. The monocytes were round, with a basophilic and sometimes vacuolated cytoplasm that contained non-specific esterase activity. The neutrophils were large and round, with typical neutrophilic granules that sometimes showed moderate staining. The nuclei were rod-shaped and occasionally segmented, with PAS-positive granules that gave a weak reaction for peroxidase. The heterophils were large and round with coarse eosinophilic and basophilic, PAS-positive granules. The eosinophils were round and medium-sized, with eosinophilic granules that generally gave a negative reaction in all cytochemical stainings. The marked variation in the granulocyte morphology of H. littorale meant that a standard analysis based only on the morphology of these cells was insufficient for identifying all of the cell types.