Landscape management has altered the population dynamics of many species residing in old growth ecosystems. As a result, fragmentations in residual old oak habitats in Sweden have led to the decline of endangered and long-lived polypores including the bright orange Hapalopilus croceus. Since little is known about this species, the aim was to investigate its growth, mating, and population biology using molecular and ecological methods. Specifically, what its growth patterns are, what its mating type is, and whether there is genetic variation among sampled populations in Sweden and the Baltic area. Fruiting body samples were collected from 34 localities in Sweden and 6 localities in neighboring Baltic States. Heterokaryotic mycelia were then cultivated from these samples and the genomes sequenced. 42 single-spore isolates from SLU's culture bank were also revived and cultured. These homokaryons were then crossed to determine mating type. Mating type was examined using morphological observations and calculated via mating matrix. To examine growth rate and establishment, H. croceus mycelia were grown in comparison to Fomitopsis pinicola mycelia. To explore the population biology, the potential for gene flow was analyzed among Swedish and Baltic isolates. As a result, H. croceus was found to be slow growing, exhibit a tetra polar mating type, and with limited genetic variation in the Baltic Sea area. Its slow growth could provide a possible insight into colonization strategy, whereas its multiallelic mating system could possibly increase outcrossing in the future. Minimal genetic variation could result from previous connectivity between oaks, where the fungus may have outbred readily and colonized more hosts, maintaining a more uniform genetic structure. However, recent habitat alteration has not been analyzed for this species, nor has community interaction among other fungi and fruiting dynamics. Thus, further examination of these factors would help expand this study and possibly provide conservation and management guidelines for future projects.