During the myotomal myogenesis in pike (Esox lucius) two phases of muscle differentiation can be distinguished. In the first phase, the somite cells-derived stock, the primary myoblasts (of mesodermal origin), fuse to form multinucleate myotubes. Participation of myotomal cells of mesodermal origin is insufficient for further muscle development. In the second stage mesenchymal cells migrate, via myosepts, into the myotome between myotubes. Immunocytochemical detection of proliferating cell nuclear antigen (marker of S phase of cell cycle) showed their mitotic activity. Transmission electron microscope analysis revealed that the differentiation of these cells depends on their position. Cells remaining in the myosepts develop into fibroblasts and produce collagen fibres, while those that have migrated into the myotomes transform into secondary myoblasts. Mesenchymal cells in the studied species are believed to participate in hypertrophy and hyperplasy of muscle fibres. Thus the muscle fibres in pike (E. lucius) are of mesodermal-mesenchymal origin.