The regeneration of longitudinal muscle bands (LMBs) in the sea cucumber Stichopus japonicus was studied using light and electron microscopic and immunocytochemical methods. Previous investigations of holothurian organs showed the presence of some cytoskeletal proteins which were specific for LMBs only. One of them, the 98 KDa protein, was isolated by means of SDS-electrophoresis and used as an antigen to obtain polyclonal antibodies. When tested on paraffin sections of sea cucumber organs, the antibodies were shown to interact only with coelomic epithelial cells covering the LMBs. The antibodies were used to study LMB regeneration after transverse cutting. During regeneration no signs of myocyte dedifferentiation or mitotic division were observed. In the wound region, damaged myocytes degenerated and muscle bundles desintegrated. However, the coelomic epithelial cells dedifferentiated and began to invade the LMB. Just beneath the surface these cells formed clusters (muscle bundle rudiments). The number and size of the clusters gradually increased, the cells lengthened and developed contractile filaments. These observations suggest that new muscle bundles arise from coelomic epithelial cells covering the LMBs. The migration of coelomic epithelial cells into the damaged LMBs and their myogenic transformation are the basic mechanism of holothurian muscle regeneration.