The ultrastructure ofHolothuria poliicoelomocytes is described and related to cell functions. Amebocytes, spherule cells and progenitor cells are the major circulating cellular types. Two morphological phases (bladder and filipodial) of the amebocytes are distinguished. Type I and Type II spherule cells are described on the basis of the morphological and ultrastructural characteristics of their granules. Maturative stages of these cells are also observed. Probably a third type of spherule cell is present. It could correspond to the acidophilic and Schmorl's positive cells. Progenitor cells lacking elaborate cytoplasmic specialization and with a probable staminal function are also normal circulating coelomocytes ofHolothuria polii.