Morphological studies suggest that there are several types of decapod hemocytes; however, distinguishing criteria based on conventional staining techniques are often subtle or ambiguous. Cytochemical features of ridgeback prawn (Penaeidae: Sicyonia ingentis) hemocytes were studied using specific stains for lysosomes, cytoplasmic contents, and granule enzymes. This approach facilitates the differentiation of cell types in the ridgeback prawn and provides information on the functions of and relation ships among different cell types. Agranular hemocytes and a subgroup of small granule hemocytes contain extensive cytoplasmic glycoprotein deposits which display smudgy, intense staining with Sudan black B. As previously shown, coagulogen-the clotting material in decapods-stains with Sudan black B when extracted from lysed hemocytes. Other hemocyte types display light staining limited to granule membranes. Lysosomes are not observed in agranular cells and are rarely present in small granule hemocytes with glycoprotein deposits. Small granule hemocytes without deposits and large granule hemocytes contain numerous lysosomes as shown by the presence of acid phosphatase, β-glucuronidase, and nonspecific esterase. Acid phosphatase is observed in the Golgi body of these cells, within small vesicles, and in small granules. The granules in large granule hemocytes rarely show acid phosphatase reaction, yet small acid phosphatase-positive vesicles fuse with the large granules. The acid phosphatase in the large granules may exist in an inactive form. Prophenoloxidase activity is localized only in large granules. The physiological significance of hemocyte cytochemistry is also discussed.