Intraspecific planting and introduction of new species of fish has been much more restricted in northern Scandinavia than in many other parts of the world. Further, the general geography, with a large number of independent drainages and numerous impassable waterfalls, often prevented extensive spread of planted freshwater fish. Therefore, the genetic patterns observed among Scandinavian populations, e.g. their structure and the amount of distribution of genetic variation, may more accurately picture a natural situation than that reflected by stocks which have been influenced by a variety of fishery management activities. Natural populations of Scandinavian salmonids seem to be characterized by a more apparent subdivision than is usually observed, and multiple cases of genetically distinct sympatric demes have been identified within many species. Detailed electrophoretic analyses of natural populations have revealed the existence of previously unrecognized reproductive units exhibiting quite different ecological and morphological characteristics. At least in the brown trout (Salmo trutta), these units seem to have evolved independently, and there are no indications that they reflect the existence of two or more widespread sub- or sibling-species. In several cases attempts to preserve, in hatcheries, the genetic characteristics of endangered or unique remnant natural populations have proven unsuccessful; genetic changes and loss of genetic variability have occurred during the stocking. There is also strong evidence that planted hatchery fish have hybridized and drastically altered the genetic composition of valuable remnant natural populations. We discuss the ecological and fishery management implications of these findings and indicate some of the problems which we consider important to be addressed in future research.Key words: genetic characteristics, Scandinavian salmonids, sympatric demes, identification, conservation