AbstractThe blood cells of a solitary ascidian,Halocynthia roretzi, were examined by electron microscopy (EM) with reference to their appearance by light microscopy (LM). In addition, their movement and stainability by vital dyes was observed by phase‐contrast microscopy, and their stainability by Giemsa was also examined. Nine cell types were recognized: vacuolated cells, hyaline amoebocytes, small amoebocytes, granular amoebocytes, macrogranular cells, globular cells, lymphocyte‐like cells, large basophilic cells and large granular cells. Vacuolated cells were found to possess various numbers of vacuoles containing strongly electron‐dense materials and could be divided into at least three subgroups. Granular amoebocytes contained microfilaments and many granules of uniform size. Hyaline amoebocytes and small amoebocytes seemed to be specialized as phagocytes. Macrogranular cells and globular cells were not well characterized. In the blood of adult individuals, hemoblasts were rarely found, although lymphocyte‐like cells were present. Each of two large cells, large basophilic cells and large granular cells, possessed novel granules or vacuoles, whose functions remain to be elucidated. The possible functions and relationships of these cells among various ascidian species are di