The micromorphology of decay in wood caused by a certain group of single-cell bacteria, called tunnelling bacteria (TB), is described. TB are characterised by their tunnelling ability within the secondary cell walls of wood fibres. Pure cultures of TB have not yet been obtained although the typical decay patterns can be reproduced using mixed bacterial cultures. Some indications of lignin degrading ability by TB have also been found. Laboratory studies indicate that TB are able to attack a wide variety of wood species which are more or less resistant against other forms of decay, either because of preservative treatment, chemical modification, high lignin content or presence of natural toxic extractives. Little information exist at present on the importance of TB for timbers in the field. Most reports are concerned with TB decay of wood in marine or fresh water environments.