Patterns of bacterial decay have been examined using light and scanning electron microscopy. Two morphologically distinct patterns have been recognised in preservative treated eucalypt power transmission poles. Bacterial Decay I (BDI) involves the removal of the secondary wall layers initiating with erosion of the S3 layer from the cell lumen. As decay progresses the S2 layer and S1 layer are degraded until only the primary wall middle lamella complex is left intact. Bacterial Decay II (BDII) appears to be initiated in the S2 layer of the secondary wall. Characteristic cavities are formed in the S2 layer with the S3 layer appearing to be unaffected. Coalescence of the S2 cavities results in the S3 layer and primary wall-middle lamella complex being the only cell wall components remaining. Both BDI and BDII have been observed in CCA and PCP treated eucalypt poles, situated in a variety of natural environments throughout Queensland. The significance of the widespread occurrence of bacterial decay is discussed. In addition tunnelling bacteria type decay patterns have been observed in CCA treated Pinus elliottii Englm. test stakes.

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