Essential for muscle fiber formation and hypertrophy, muscle stem cells, also called satellite cells, reside beneath the basal lamina of the muscle fiber. Satellite cells have been commonly identified by the expression of the Paired box 7 (Pax7) due to its specificity and the availability of antibodies in tetrapods. In fish, the identification of satellite cells remains difficult due to the lack of specific antibodies in most species. Based on the development of a highly sensitive in situ hybridization (RNAScope ® ) for pax7 , we showed that pax7 + cells were detected in the undifferentiated myogenic epithelium corresponding to the dermomyotome at day 14 post-fertilization in rainbow trout. Then, from day 24, pax7 + cells gradually migrated into the deep myotome and were localized along the muscle fibers and reach their niche in satellite position of the fibres after hatching. Our results showed that 18 days after muscle injury, a large number of pax7 + cells accumulated at the wound site compared to the uninjured area. During the in vitro differentiation of satellite cells, the percentage of pax7 + cells decreased from 44% to 18% on day 7, and some differentiated cells still expressed pax7 . Taken together, these results show the dynamic expression of pax7 genes and the follow-up of these muscle stem cells during the different situations of muscle fiber formation in trout.