The purpose of this study was to develop polymorphic site index curves for the regeneration of coigüe (Nothofagus dombeyi) in the pre-Andean mountains of the IX Region of Chile. Three forest sites were defined a priori for this purpose, based on a spatial analysis of physiographical variables related with ecological requirements of the species: elevation, aspect and gradient. Subsequently, for the purpose of adjusting the age-height models, an analysis was carried out in 50 sample units, together with a dendrometric sampling of 150 felled dominant and co-dominant trees. The results allowed the determination of three forest sites, with indexes of 20, 18 and 16 meters for a key age of 40 years. Aspect and elevation were both significant variables affecting site index.