An in situ screen for genes expressed in the skeletal muscle of eyed-stage trout embryos led to the identification of a transcript encoding a polypeptide related to CILP1, a secreted glycoprotein present in the extracellular matrix. In situ hybridisation in developing trout embryos revealed that CILP1 expression was initially detected in fast muscle progenitors of the early somite. Later, CILP1 expression was down-regulated medio-laterally in differentiating fast muscle cells, to become finally restricted to the undifferentiated muscle progenitors forming the dermomyotome-like epithelium at the surface of the embryonic myotome. At the completion of somitogenesis, CILP1 expression was concentrated in the myoseptal/tendon cells that develop between adjacent myotomes but was excluded from the skeletogenic cells of the vertebral axis to which the most medial myoseptal/tendon cells attach. Overall, our work shows that muscle cells and myoseptal/tendon cells contribute dynamically and cooperatively to the production of CILP1 during ontogeny of the trout musculoskeletal system.