Morphological characteristics of hemocytes in four copepods（Copepoda, Maxillopoda, Crustacea）were histologically examined by light microscopy. Only a single type of hemocyte, basophilic granulocyte, was observed in the hemal space of two parasitic copepods, Pseudocaligus fugu（Siphonostomatoida）and anchor worm Lernaea cyprinacea（Cyclopoida). On the other hands, Tigriopus japonicus（Harpacticoida）and Cyclops vicinus（Cyclopoida), which have free life style, lacked circulating hemocytes, however, hemocyte-like granular cell was occasionally observed in/on the tissues. The granular cells also contained basophilic granules as hemocytes of parasitic copepods. For comparison, other crustaceans, water flea Daphnia pulex（Cladocera, Diplostraca, Phyllopoda, Branchiopoda）and Sacculina confragosa（Kentrogonida, Rhizocephala, Thecostraca), were also examined. The water flea has chromophobic granulocyte, but no hemocyte and granular cell was observed in S. confragosa. These observations indicate that P. fugu, anchor warm and water flea belong in onohemocytic group, but T. japonicus, C. vicinus and S. confragosa in hemocyteless group. Furthermore, hemocyteless crustaceans were classified into two subgroups, euhemocyteless group (S. confragosa) and pseudohemocyteless group（T. japonicus and C. vicinus), according to lack/occurrence of the hemocyte-like granular cell.