The coelomocytes suspended in the coelomic fluid and occurring in the coelomic epithelial layer of the sea cucumber Apostichopus japonicus (Selenka)(Holothuroidea: Aspidochirota: Stichopodidae) function as mediators of the immune system, trephocytic cells and nutrient transport cells. Types of coelomocytes are characterized based on their morphological and ultrastructural features. Flow cytometry plus light and electron microscopic analyses were conducted in order to characterize the coelomocytes of A. japonicus. Six types of coelomocytes were identified: lymphocytes, morula cells, amoebocytes, crystal cells, fusiform cells and vibratile cells. Within these major categories, several distinctive cell types occurred that might represent developmental stages. The mean±SD coelomocyte concentration in the individuals (body length: 10 to 15 cm; weight: 100 to 150 g) was (3.79±0.65)× 10 6 cells ml-1. The coelomic fluid contained mainly hyalinocytes (76.69%) and granulocytes (23.31%).