The lateral musculature of elvers is differentiated into two fibre type on the basis of alkaline-labile (pH 10.2) myofibrillar ATPase activity. Slow muscle forms a relatively homogeneous fibre population, whereas fast muscle shows a heterogeneity with respect to both fibre size, and position in the myotome. The low aerobic capacity of slow fibres reflects the energetic requirement of anguilliform locomotion. A morphological continuum of myogenic cells occurs within mature; differentiated myotomal muscle similar to that described for embryonic myogenesis. No evidence could be found for regional growth nodes. Small fast fibres (< 100μm2) represent immature, but differentiated fibres undergoing hypertrophy. There is considerable variation in the capillary supply to both fast and slow muscle, and between homologous muscle off different species. Methods are described to determine the minimum sample size required for a stable, reproducible parameter estimate, and to assess the orientation of the capillary network. This is shown to be highly anisotropic. Capillary volume and surface densities are thought to be the most appropriated indices to use with fish muscle. The springs migration of elvers is shown to be a mixed population, of similar annual composition. The main migratory wave are true post-metamorphic, juvenile eels. There is a partial (Precht type 3) compensation in VO2 on acclimation to 10° and 29°C. The extent of the physiological acclimation reflects the environmental constraints of the migration. Differences in structure and complexity of multiple and focal innervation were investigated using fast muscle from representative teleosts. Endplate structure is similar in both types. Cod ventral spinal nerves have fewer motor, but more sensory axons than homologous nerves in eel. A novel way of visualizing the extent of inter- end intra segmental branching of nerves, intracellular marking of nerve routes with cobalt, reveals extensive branching in cod mytomes and cross-innervation between at least 3 segments. In cel, branching is restricted to a single mytome. These results reflect the mechanical and nervous control over the locomotory waveform.