During its anadromous migration, the Atlantic salmon, Salmo salar, shows morphological transformations that affect various parts of its skeleton. The most spectacular concerns the jaws that lengthen and thicken to form a “kype” in the males. During this time, scales and vertebrae show important erosion in the males as in the females. The aim of the present work was to verify if the dentary has a positive osseous global assessment: what is the true balance between osteogenesis and resorption during the anadromous migration? An analysis of the dentary mineralising rate (DMR), the dentary bone compactness (DBC) and the mineralization degree (MDD) of dentary bone on fish caught in the Scorff River (Brittany, France) at the beginning of their ascent and after spawning, shows two main phenomenons: a) a conspicuous increase of the bone compactness; this means that the quantity of osseous substance during the anadromous migration, in males as in females, is higher. This is probably the result of an increase of osteogenesis, but it also could be a decrease of bony destruction by osteoclastic cells or, even a grouping of both processes; b) a small loss of mineral: the TDM decreases. This loss can have two origins: i) an incomplete mineralization of new bone (the maturation of hydroxyapatite crystals is not achieved); ii) a diffuse demineralization (or halastasy) of old bone. An osteoclastic activity releases both organic components and mineral ions that give a null balance for the TDM, whereas the halastasy attacks only the mineral component. In fact these two hypotheses (unmatured new bone and halastasic old bone) can act in synergy. Lastly, the evolution of the histological characteristics of dentary bone are practically the same in the two sexes except for the mineralization degree that is slightly higher in the females than in males.