Wild Spanish populations of Salmo salar were analyzed for their variability at the MEP-2* locus that determines the malic enzyme expressed in muscle mitochondria. The allele frequencies of these populations have been found to be associated with sea-age of returning adults and freshwater temperature; in a minor extent with foreign stocking of Irish and Scottish alevins; and finally, no association was found with precocious parr maturation.Natural populations are often assumed to be in equilibrium; nevertheless, HardyWeinberg conditions are rarely reached in any population. Genetic equilibrium is maintained by a delicate balance between several forces such as selection, genetic drift and gene flow that can promote major changes of the genetic composition (gene and genotype frequencies) of populations. These factors are critical in small or declining populations, in which a little change could result in extinction.A case of a species with populations bordering extinction is the Atlantic salmon (Salmo salar L.) in many Spanish rivers, such as the Asturian ones, located in the central area of Northern Spain (Figure 1). The headwaters of these rivers are placed in a cline of heights following the Cantabric mountain chain, decreasing from East to West, a fact that displays a longitudinal cline of water temperatures (mean summer temperature ranging from 12.6 ºC in the Cares to 16.4 ºC in the Esva in 1990; data from official routine records of the Confederacion Hidrografica del Norte). The stocks of these rivers typically consist of a majority of two-sea-winter salmon with variable proportions of one-sea-winter grilse (Nicieza and Braña, 1993). Placed on the southern limit of the European distribution of this species, the sizes of Spanish populations have decreased dramatically during this century, having been reduced to a few adults in many cases. The reduction of population sizes is