The molecular characterization of the hyperplasia and hypertrophy that characterize postembryonic muscle development in rainbow trout is of great interest to aquaculturists because of the commercial value of the species. Determination of temporal expression levels of the genes that control muscle development is an important step in molecular analysis. Real-time quantitative reverse transcriptase polymerase chain reaction was used to characterize expression in the muscle of 3 MRF, 2 MEF, and 2 myostatin genes during 9 stages of trout development. Expression of genes that promote muscle growth (MRF and MEF) peaked in swim-up fry, and in some cases again in 25-g, 140-g, and spawning fish. Myostatin genes, which restrict muscle growth, were expressed at very low levels early in development, but their expression levels were elevated in 140-g and spawning fish. Expression levels and the known function of each tested gene were used to infer the extent of hyperplasia, hypertrophy, and restriction of muscle growth during each stage. Both hyperplasia and hypertrophy appeared to peak in swim-up fry and spawning females, and hyperplasia also appeared to peak in 25-g fish. These results should provide valuable information for developmental biologists and those interested in understanding muscle growth in fish.