Description: The re-initiation of bone development in adult starving Atlantic salmon (Salmo salar) during their energetically expensive upstream migration is remarkable and deserves closer examination. Dramatic alterations of the skull bones and teeth, most prominently, the development of a kype in males, are widely known but little studied or understood. We describe the microstructure and the cellular processes involved in the formation of the skeletal tissues of the kype. Fresh bone material, obtained from animals migrating upstream was subjected to radiological, histological or histochemical analysis. We show that the kype is, in part, composed of rapidly growing skeletal needles arising at the tip of the dentary. Proximally, the needles anastomose into a spongiosa-like meshwork which retains connective tissue inside bone marrow spaces. Ventrally, the needles blend into Sharpey fiber bone. Skeletal needles and Sharpey fiber bone can be distinguished from the compact bone of the dentary by radiography. Rapid formation of the skeleton of the kype is demonstrated by the presence of numerous osteoblasts, a broad distal osteoid zone, and the appearance of proteoglycans at the growth zone. The mode of bone formation in anadromous males can be described as 'making bone as fast as possible and with as little material as possible'. Unlike the normal compact bone of the dentary, the new skeletal tissue contains chondrocytes and cartilaginous extracellular matrix. Formation of the skeleton of the kype resembles antler development in deer (a form of regeneration), or hyperostotic bone formation in other teleost fishes, rather than periosteal bone growth. The type of boneformation may be understandable in the light of the animals' starvation and the energetic costs of upstream migration. However, the structured and regulated mode of bone formation suggests that the skeleton of the kype has functional relevance and is not a by-product of hormonal alterations or change of habitat.