Decaying Betula pendula Roth trunks (measuring about 6 m in length) with brown rot or white rot were analyzed for the followings: stage of decay, wood structure, porosity, chemical composition of decayed wood, spatial distribution of fungal biomass, and mycoflora diversity. The fungal biomass in the trunks, although heterogeneous, is higher in white rot than in brown rot. As expected, the lignin/holocellulose ratio is higher in brown rot than in white rot (maximum 14.83 versus 0.67). Brown rot is generally more porous than white rot and presents a higher water absorption capacity but it retains less air. The results show that the basidiomycetes mycoflora is active in the white rot decay process. However it is not involved in the brown rot decay process because the wood is already much decayed (as much as 80% and more of lignin content). With the exception of moulds, the only mycoflora that could be isolated repetitively from the brown rot station was in fact pockets of white rot. Moreover, in brown rot, none of the epiflora matched the isolated endoflora. Keywords: white rot, brown rot, mycoflora, biomass.