Samples of coelomic epithelium and coelomocytes suspension of intact and wounded starfishes Asterias rubens L. were analyzed by electron microcopy. It has been demonstrated that coelomic epithelium is composed of three types of cells: flagellar (approx. 60%), secretory (approx. 3%) and myoepithelial (approx. 37%). Flagellar and secretory cells form the apical surface of coelomic epithelium. Secretory cells are represented by two subtypes: granular and mucous secretory cells. Myoepithelial cells are located in the basal zone of the epithelium. Adjacent flagellar cells are separated by intercellular gaps of various size in 4-5% of cases. These gaps are apparently the lacunae left by the flagellar cells after their departure to the coelomic cavity. The morphological pattern of transition of coelomic epithelium flagellar cells to the coelomocytes has been characterized. No significant structural alterations in organization of the coelomic epithelium were revealed after moderate wounding used in the present study. Small round-shaped young coelomycytes (approx. 3%) and bigger mature coelomocytes (approx. 97%) were found in coelomocytes suspension. A flagellum was revealed on the surface of one of the young coelomocytes. Surface of the mature coelomocytes forms the processes of various size and structure; their cytoplasm contains lysosomes and fagocytic vacuoles of different size. After wounding, activation of coelomocytes was noted finding expression in the sharp rise in the number and the length of their surface filopodia, and in the multicellular aggregates formation. By the sum of the ultrastructural data, histogenesis of coelomocytes from the flagellar cells of the coelomic epithelium is supposed to be a process of cellular transdifferentiation.