The structure of the olfactory epithelium in developing Arctic charr, Salvelinus alpinus, was studied by light and scanning electron microscopy. Embryos, larvae, free-swimming fry, and individuals up to the age of 96 months after hatching were studied. In larvae about 10 days after hatching, microvillar and ciliated olfactory receptor cells were located in a pit and no nares were differentiated. Fifty days after hatching, two nares were present on each side of the head, and about 20 days later one or two lamellae had started to differentiate and were visible as an elevation in the caudal part of the floor of the olfactory chamber. Six months after hatching there were 5 – 10 club-shaped lamellae, which in older fish acquired a more disk-like appearance. The number and size of lamellae increased with the size of the fish, reaching the maximum number, 10 – 15 per rosette, 18 – 30 months after hatching. At 18 months, secondary folding of the lamellae had started. Eighteen months later, differentiated secondary lamellae were present and most of the central raphe was composed of indifferent epithelium. The olfactory receptors were located in the depressions between the secondary folds. The development of the olfactory organ is discussed in relation to the results of behavioural studies.