Viable hemocytes of the common tunicate Styela clava are classified into four groups designated as eosinophilic granulocytes, basophilic granulocytes, hyaline cells and lymphocyte-like cells. Eosinophilic granulocytes, actively amoeboid, have large refractive granules that stain with neutral red. Basophilic granulocytes do not stain with neutral red and formed couplets or triplets. Hyaline cells, which often contain phagosomes, have electron-dense small vesicles recognizable only by electron microscopy. Hemoblasts have a characteristic large nucleolus which is visible by light microscopy. Eosinophilic granulocytes and hyaline cells actively ingest yeast particles in vitro. This classification simplifies former ones by correlating electron microscopy, with light microscopy, and viable with fixed hemocytes. Clearly viable tunicate hemocytes can be identified by simple methods. We have provided clear and more accurate descriptions which will lessen the controversy often associated with assigning hemocyte functions in immunodefense responses both in vivo and in vitro.