Skeletal muscles of developing pectoral fins in rainbow trout larvae (Salmo gairdneri) were analyzed by electron microscopy. Large, branched mitochondria were dominant structures in developing myotubes. Mitochondria were associated with the tubular system (T and SR). New mitochondria arose from old ones when the latter extruded whorls of paired membranes surrounding a nonmembranous core. The core was comprised in part of a dense material, presumably, DNA. The developing muscles were characterized by two sets of caveolae which provided the major contributions to the tubular system. Large caveolae gave rise to elements traditionally designated as SR tubules but which later lost their exterior connections. Small caveolae gave rise to small diameter tubules that appear to be analogous to T tubules, which maintained connections with the exterior. Both tubular elements abutted mitochondria. The two elements ran parallel to each other and intersected with each other to form junctions. Each set of elements possessed intratubular junctions.