Fish flesh is composed of muscle, conjonctive and adipose tissues. Muscle and conjonctive tissues are supposed ta play a significant role in the texturai properties of the flesh. Muscle experience large qualitative and quantitative changes during development including durîng late development in large commercial size fish. This paper try to related these changes with flesh quality variability. Skeletal muscle could be divided in a superficial red muscle and a deep white muscle. The contractile and metabolic characteristics of these two muscles have been extensively studied in a lot of fish species. We analysed further the variabilities of some of these characteristics within each muscle. Different criteria which described the physical, biochemical and metabolic characteristics of muscle have been developed. The validity of these criteria for the assessment of variability of flesh quality was tested in rainbow trout and brown trout.