The aim of this research was to investigate the microbiological activity in forest soils from four different sites in Serbia. Microbiological activity was affected by seasonal changes and tree species. The highest total number of bacteria and actinomycetes were measured on Stara planina site where beech tree dominates. The number of fungi increased in autumn on Stara planina and Kopaonik sites, where the presence of beech in stands was dominant. Number of cellulolitic microorganisms were 2-3 times lower in all investigated sites in the autumn than in the spring. The highest dehydrogenase activity was measured in stands on Stara planina.