Abstract: The internal defence system ofApostichopus japonicus is dependent on circulat ing coelomocytes and blood cells, and in recent years, the studies havem ainly concerned them orpholog ical characterization of different coelomocyte types.However, heretofore, there is not an uniform criteria on classification of coelomocytes.In addition, studies on blood cells inApostichopus japonicus were very few, just somebody conjectured that coelomocytes and blood cellswere identical and blood was smi ilar to coelom ic fluid. In order to investigate the characterization of coelomocytes and blood cells inApostichopus japonicus and ascerta ined the smi ilarity of above both, in th is paper, morphology and structure of coelom ocytes and blood cells inApostichopus japonicus were described on the observation of liv ing coelomocytes stain ing, biology slice(blood vesse,l retem irab ile and intest ine-and ultrath in sections(coelom ocytes and blood cells-and the count of coelomocytes were studied. T he results showed thatm orphology and structure of coelom ocytes and blood cells were identical.T he coelom ocytes could be div ided into s ix types: Lympho id cells(5-6Lm, spherical shape with 6-7Lm pesudopods and a higher nuclearcytoplasm ic rat io), Spherulocytes(5-13Lm, spherical shape and a few granules in cytoplasm), Am oebo id phagocytes (5-10Lm, irregular shape with several pesudopods), H yaline cells(7-10Lm, spherical shape and a low er nuclear-cytoplasm ic ratio), Fus iform cells(fus iform, 6-12Lm of length-diam eter and 1-3Lm o f short-diam eter-and Crystal cells (5-10Lm, polyhedra,l rectangle on side view and hexagon on apical view).The Sphorn locytes could be div ided into ?(5-10Lm with no sports ability), ?7-12Lm with several pesudopods on radial arrangem ent-and ?types(6) 13Lm with several spherical refractive bodies in cytoplasm). In add ition, all coelomocytes except crystal cells were observed in blood vessel.On the bas is of form and structure, we thought the ontogenesis of coelomocytes and blood cellswere probably same.How ever, the certain relationship of both two should be further verified in future us ingm onoclonal antibodies.Not only in coelom ic fluid but also in blood, the count of Lympho id cells, Sphorulocytes and Am oebo id phagocyteswere the most, follow ed by Hyaline cells, Fusiform cells and then ofC rystal cells.The total coelomocyte count(TCC-and differential coelom ocyte count(DCC)were determ ined six tmi es in March, May, July, August, November 2005 and January 2006 atDalian, respectively.The results showed that the annua l average of total coelomocyte countwas(3.85?.21-@ 106/mL, the count of Lymphoid cel,l Sphorulocytes and Amoebo id phagocytes was(1.58?.11-@ 106/mL((41?.47)% ofTCC),(1.16?.13-@ 106/mL((30?.89)% ofTCC),(0.99?.07-@ 106/mL((25?.98)% of TCC), respect ively.The total number of above three k inds of cells was account for(96?.53)% of TCC.