The activity of Fomes fomentarius (L.) J. Kickx f. also known as Tinder fungus is usually perceived negatively in forest ecosystem because it is causer of active tree´s rot. Is this activity perceived the same way in the forests? The answer is contributed by our own results of the research in two forest localities, Low Tatra (Obrštín and Pod Latiborskou hoľou) in 2017- 2018. Through fructification (presence of 263 fruiting bodies) we found out that Fomes fomentarius is mostly colonizing the wood of breech Fagus sylvatica L. We evaluated 814 trees or their parts in those two localities. The total volume of necromass was determinate for 550 cubic metres. Necromass colonized by tree fungus Fomes fomentarius represented 330 cubic metres i.e. 59% of total volume of dead wood. In comparison of dead wood and wood of active trees it significantly colonized less (from 264 active trees only 9 were colonized). The results are indicating that Fomes fomentarius (l.) J. Kickx. in primeval forests do not need to present fytopathological harm vice versa it is participating in biology-ecology stability.