The heartwood of Belian (Eusideroxylon zwageri), an East Malaysian timber species, is naturally durable due to its certain unique anatomical features and high content of extractives. The timber can tolerate years of exposure to hazardous conditions in ground contact and other situations without any significant loss in its strength. A few Belian transmission poles sampled from one locality in Sarawak showed only surface heartwood decay at the groundline after 20 years. Transmission electron microscopy (TEM) revealed the presence of soft rot type cavities in fibre walls. In addition to soft rot attack of secondary walls, in severely decayed tissues, other areas of fibre walls, including the highly lignified middle lamellae, were also degraded. Bacteria were also abundant in these cells, suggesting that disintegration of wood tissues at the surface could result from a combined action of soft rot fungi and wood degrading bacteria.

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