Land-locked populations of Arctic charr in four lakes on Northern Ellesmere Island (80° N) were found to consist of two distinct sizes: ‘dwarf’ and ‘normal’ charr. Both groups attained sexual maturity but differed in appearance and habitat. The smaller fish, occupying the more marginal habitats, retained their parr-markings; the larger group had the general characteristics of smolts, being more silvery and without parr-marks. In their juvenile stages, the charr destined to attain the larger group were indistinguishable from members of the smaller group. Although fish in the larger group were capable of cannibalism, this was rarely observed. In general, the fish in the larger group were older than the smaller ones but great variation in size at a given age resulted in certain age classes containing representatives of both groups. The population structure varied considerably between lakes; a high proportion of ‘normal’ charr correlated well with a high growth rate in the first few years of development. It is postulated that the two groups live in dynamic equilibrium where the advantages of progenesis (retention of juvenile characters by adults) in the smaller type are traded against the larger proportion of the energy resources available to the larger type. The concept of heterochrony in an ecological setting is introduced.