Salmonids undergo a developmental transition from parr to smolt that involves a number of physiological and morphological changes. In recent years, my laboratory has studied shifts in red muscle function at this parr-smolt transformation (PST) in rainbow trout, Oncorhynchus mykiss. Parr red muscle has faster contraction kinetics than smolts, including faster rates of activation and relaxation and a faster maximum shortening velocity. At PST, a transition in swimming behavior is also observed, with lower tailbeat frequencies and longer EMG duty cycles in the older smolts. Lastly, there is molecular correlate to changes in kinetics and behavior. During PST, there is a developmental reduction in the number of myosin heavy chain (MHC) isoforms in the red muscle of rainbow trout. Since MHC composition of muscle can determine contractile properties, these molecular results suggest a mechanism for the transition in red muscle kinetics and steady swimming. The red muscle of parr is more likely to contain the fast-twitch or white isoform of MHC, resulting in faster contractile properties of that muscle and higher tailbeat frequencies during steady swimming. Lastly, experimental work supports the conclusion that the shift in kinetics causes the observed shift in swimming behavior.