In this study, Penaeus monodon hemocytes were identified and characterized on the basis of morphology, labeling by monoclonal antibody markers (mAbs Z5E10, Z6A5, Z6A6, and Z6H8), and cellular defense reactions. Using light and transmission electron microscopy (LM and TEM), three hemocyte types were identified based on the number and size of the cytoplasmic granules and the nucleus-to-plasma (N/C) ratio: hyaline cells (HC) are round or ovoid small cells with a high N/C ratio (0.58–0.77), and although they are clear under LM, a small number of granules can be seen with TEM; semigranular cells (SGC) are irregular in shape with medium-size granules (0.31–0.9 µm); and granular cells (GC) are round or ovoid in shape, and contain the most and the largest (0.54⁻1.9 µm) granules. Immunofluorescence staining showed that separately the four mAbs labeled some of the HC, SGC, and GC. Results from double staining with Z6A5 and Z6H8 showed that all three types of shrimp hemocytes could be divided into two subclasses, one labeled by both mAbs (Z6A5 Z6H8 ) and the other labeled by neither (Z6A5⁻Z6H8⁻). Beads (0.5 µm in diameter) were engulfed by both HC subclasses, but not by any of the SGC or GC. Acid phosphatase and phenoloxidase were expressed only in some of the Z6A5⁻Z6H8⁻ hemocytes, and not in any of the Z6A5 Z6H8 cells. These results suggest that cellular morphology and two of the four mAb markers (Z6A5 and Z6H8) can be used to classify P. monodon haemocytes into six functionally distinct subclasses.