Chemical composition and size of fibre and adipose cells in the flesh of rainbow trout were studied. The flesh of fish included a superficial red muscle which is composed of small fibres (beta red type) and a deep white muscle which is the main tissue and is composed essentially of a mosaic of small and large fibres (alpha white type). The mean size of muscle fibres increased with trout body weight. The amount of collagen in the flesh was low (below 1% of fresh muscle weight) and thermosoluble collagen seemed to be more variable than acidosoluble collagen. Thus, the quality of the flesh would depend on the characteristics of muscle tissues. Lipid content in the flesh of trout of market size reared in fresh water was low (below 10% of fresh weight) and it was located in abdominal and dorsal subcutaneous adipose tissues. Adipose tissues were generally composed of two populations of adipose cells, the small adipose cells (20/30 microns) being present in developing adipose tissue and large adipocytes whose size increased with the lipid content of the flesh.