The structure of hemocytes from two species of penaeid shrimp was examined by light and electron (TEM) microscopy. Hemocytes from the two species are indistinguishable and are classified as either agranular, small-granule, or large-granule hemocytes. Agranular hemocytes are the smallest of the hemocytes, lack granules, compose only 5-10% of the circulating hemocytes, and are nonrefractile when examined by light microscopy. Small-granule hemocytes are the most abundant type of hemocyte (75% of all hemocytes), appear nonrefractile, and contain a variable number (1-40) of granules (0.4 ΞΌm diameter). Large-granule hemocytes compose 10-20% of the hemocytes. They are filled with granules (0.8 ΞΌm in diameter) that are highly refractile when examined by light microscopy and are electron-dense when examined by TEM. Our classification scheme is based solely on the absence or presence and relative size of granules. Features used by other researchers, such as cell size, shape, and staining properties, were not used because these features are subtle and/or subjective. The proposed classification is compared with schemes developed for other decapods, and its usefulness and limitations are discussed. This scheme will serve as a basis for further studies on the maturation and physiological function(s) or crustacean hemocytes.