To identify differentially expressed genes of the white-rot fungus Heterobasidion parviporum, two cDNA libraries were constructed using suppressive subtraction hybridization (SSH) technique with RNA extracted from an advanced stage of decay area and from colonization front next to the reaction zone of the stem of a mature Norway spruce naturally colonized by the fungus. Besides several cytochrome P450s and hypothetical proteins with unknown function, the SSH libraries constructed contained, among others, genes involved in basic cellular processes, and lignin and cellulose degradation. To examine the role of selected candidate genes for each functional group, three trees, each colonized by a different genotype of the pathogen and showing a variable degree of wood decay, were used for real-time RT-PCR profiling of candidate genes. In the decay transition areas the study revealed activity centers that showed remarkable similarity in the transcript profiles of the monitored genes.