Transmission electron microscopy of decaying CCA-treated Pinus radiata timbers from an industrial water cooling tower showed the presence of a thick biofilm covering some areas of the wood. The biofilm contained various morphologically distinct forms of microorganisms embedded in a slime. The study provided evidence of the activity of soft rot fungi and tunnelling and erosion bacteria in wood cells covered by the biofilm. The extent of microbial damage to wood cells varied, with combined fungal and bacterial attack having the most damaging impact.