Rearing and transplantation experiments demonstrate that three coexisting forms of Arctic char Salvelinus alpinus (anadromous, small and large freshwater residents) belong to the same gene pool. The parr of each form segregate into all three forms. Single individuals may manifest all three forms during their lifetime, successively attaining the appearance and spawning color of each form. Males mature at a younger age and smaller size than females. Hence, in char populations featuring two or more forms, males dominate in the early maturing form and females in the late maturing form. Char populations that feature only one form (sex ratio 1:1) seem to be established through natural selection for age at sexual maturity. Segregation during the young stage depends upon their genetic constitution and access to food. Offspring of the small resident parents produced more resident individuals and fewer smolts than did offspring of anadromous parents. An increased amount of food increases the resident fraction and reduces the fraction of anadromous char. It is demonstrated that the potential for anadromy exists in populations of char in the southern nonanadromous area, and that one reason for resident behavior in the smolt may be infection of the ureter fluke Phyllodistomum conostomum Olsson.