Abstract:Tunnelling bacteria (TB) decay of the untreated and preservative treated hardwood (Betula pendula) and softwood (Pinus silvestris, Picea abies were was examined with TEM. The mechanism of bacterial entrance into the wood cell wall was studied. TB attack on the cell wall begins from the lumen side. The degradation of cell wall is caused by bacteria moving and dividing within the S1, S2 layers of the cell wall. The morphology and ultrastructure of TB and their tunnels was observed. The present work shows that the membrances of the bordered pits and simple pits appear having the capability to resist bacterial action, but bordere and the other part of cell wall are not. As TB can attack the wood treated by 0.5% Tanalith NCA, TB has a cabability of detoxic.