Gloeoporus taxicola is a widespread saprotrophic polypore that occurs on a variety of coniferous substrates in the Northern Hemisphere. In this master thesis a multi-locus sequencing approach was used, on an extensive worldwide sample, to investigate the phylogepgraphy of G. taxicola in the light of substrate affinity. Diplophase sequences fromtwo nuclear markers gave a complex phylogeographic pattern that roughly divided the specimens into two evolutionary lineages, but some admixed and highly heterozygous sequences appeared as well. To increase the resolution, cloning was performed and haplophase sequences obtained. This revealed three main clusters of haplotypes, one representing a European lineage associated with pine, while the other two had more northern circumboreal distributions, occurring on a wide number of substrates. Some specimens contained two highly divergent haplophase sequences, probably reflecting hybridization andfurther introgression between the separate evolutionary lineages. There was a strong indication of different host affinity between the two evolutionary lineages and one of the main lineages showed rather strong host specificity for a saprobic fungus. This study demonstratesthe importance of splitting heterozygous diplophase sequences into its underlying haplophase sequences to obtain a more comprehensive resolution in phylogeographic studies.