Diversity and differentiation among 11 Argentine populations of the South American southern beech, Nothofagus nervosa (Phil.) Dimitri & Milano, were studied using eight isozyme gene markers. Genetic diversity, observed heterozygosity, mean number of alleles per locus, Gregorius’ genetic distance, amount of genetic differentiation and mean level of differentiation among populations were estimated. Introgression of N. obliqua into the gene pool of N. nervosa was analysed using two species-specific isozyme gene markers (Adh-A and Pgi-B). As in most forest tree species, the main genetic variation was found within populations. Mean observed heterozygosity for the species was 17.3%, whilst mean level of genetic differentiation (δge) was 4.6%. The most differentiated locus was Mdh-B with differences in 10% of the effective number of genes. From the analysis of the species distribution pattern it is suggested that genetic variation is geographically structured through the different lake watersheds along which the species is distributed. The occurrence of natural interspecific hybridization in some of the watersheds, together with the unidirectional gene flow along these watersheds sharpens this latitudinal structure. Different evolutionary stages in the introgressive hybridization process are apparently taking place in different lake watersheds.