The aim of this paper is to present the results of the investigation on belowground biomass and its annual increment in a beech ecosystem (Calamintho grandiflorae-Fagetum) in Mavrovo National Park, Republic of Macedonia. Belowground biomass was estimated in three layers of the ecosystem (tree, shrub and herb layers) for seven years during the period 1997–2005. Allometric regressions were established for the relationship of root biomass from volume index (D²H, diameter squared × height) on a sample of 10 model trees and 13 model shrubs of European beech (Fagus sylvatica L.). Fine root biomass of trees and shrubs was estimated in soil samples to a depth of 145 cm and divided into live and dead fine roots and subdivided into thickness classes. Belowground biomass of the herb layer was assessed in 20 herb species. It was estimated that the total belowground biomass in the ecosystem was 57.75 ·ha^–1. The contribution of shrub and herb layers was insignificant (less than 0.2%). Biomass of the live fine roots was 10.16 t·ha^–1, i.e. 18% of the total belowground biomass. Annual increment of trees and shrubs was 1.03 t·ha^–1·y^–1 and 4.6 kg·ha^–1·y^–1, respectively.