A heavy metal tolerant strain of the ericoid mycorrhizal species Oidiodendron maius, isolated from roots of Vaccinium myrtillus growing in soil heavily contaminated with zinc, was previously shown to tolerate high concentrations of zinc and cadmium ions in the growth medium. We have investigated the genetic basis of this fungal strain tolerance to high zinc concentrations by using an untargeted approach. From a cDNA library constructed by using mRNA from Zn-treated O. maius mycelia, 444 clones were randomly selected and 318 were sequenced. Sequence analysis identified 219 unique clones: 117 showed homology to previously identified genes, 26 matched unknown protein coding regions found in other organisms, and 76 were novel. Variation in the gene expression level after a 20-day treatment with high concentrations of Zn was monitored on 130 unigenes by reverse northern blot hybridisation. Sixteen unigenes were shown to be either up- (9) or down- (7) regulated. The putative function of these genes and their involvement in stress tolerance is discussed.