Trunk muscle in fish, is composed of three distinct muscles (white, red and pink) which are involved in the different swimming modes (cruise or escape). Myotomal muscle has been characterized by histo- or immunohisto-chemical methods in diverse fish species, but few investigations at the molecular level have been performed. In fish, myosin, the major contractile muscular protein, presents the same structure as that found in higher vertebrates. It is a hexameric molecule constituted by the association of one pair of alkali light chains (LC2) and one pair of regulatory light chains (LCI and/or LC3) with two myosin heavy chains (MyoHC). The molecule presents a bipolar structure: the head and the rod. The head results from a globular arrangement of the N-terminal region of the MyoHC or from the proteolytic subfragment-1 associated to the light chains. The head of the molecule contains the ATP binding site and the actin site necessary to the contraction caused by a sliding of the head with regard to the thick filament formed by the rods of myosin heavy chains. The rod corresponds to a coiled-coil arrangement of two a helical myosin heavy chains. Differences in fibre types are due to different myosins with the expression of either slow or fast myosin heavy and light chains. Skeletal muscle in mammals has been shown to be composed of two slow MyoHCs (MyoHCI, MyoHCIton), four fast MyoHCs (MyoHCIIa, MyoHCIIb, MyoHCIId, MyoHCαcard) and two super fast MyoHCs (MyoHCIIm, MyoHCIIeom) (for review see Staron & Johnson, 1993). In addition, there are developmental MyoHC isoforms (embryonic, neonatal) related to muscle maturation and which appear sequentially during development (Whalen et al., 1981).