The wood rotting basidiomycete Polyporus (subgenus Leucopori Quel.) was chosen in order to investigate by genetic parameters the validity of the classical species concept based on typological characters.Species deliminations in this genus are derived from morphological characters and depend mainly on the size of hymenial pores. They were compared with those assigned from mating relations between 26 races of different geographic origin.All races could unequivocally be grouped into three separate entities corresponding with the typological species P. arcularius, P. brumalis and P. ciliatus on the basis of the following results: 1. As expected, the basic breeding system in Polyporus is the tetrapolar mechanism of homogenic incompatibility controlled by multiple alleles of the mating type factors A and B. 2. All intraspecific combinations were fertile. A conspicuous barrage formed in those crosses where dikaryotization and fruiting were impaired. This barrage is characterized by a clear zone, about 1-2 mm wide, free of aerial hyphae and reduced hyphal density in the growth medium. The delay observed in the onset of the sexual cycle was caused by retarded fusion of hyphae and blocked exchange of nuclei despite a compatible combination of the mating types. 3. Using two races of P. ciliatus as an example, it was revealed that barrage formation is started by the specific interaction of three independent genes (bi (+)/bi = barrage initiation, bfI 1/bfI 2 and bfII 1/bfII 2 = barrage formation) in a way characteristic for systems of heterogenic incompatibility: barrage formation requires the presence of the allele bi (+) in at least one mating partner additional to a heterogeneity in both bf-genes. 4. Interspecific combinations were sterile. There is no hyphal fusion between the mating partners and because of the mutual repulsion a sharp line formed in the area of contact that was designated as border line. Its formation is independent of mating type or nuclear status of the confronted mycelia. The good correspondence of the species limits derived from morphological and genetic data indicates the applicability and validity of both the typological and the biological species concept. The latter, however, proved superior in compensating the variability of morphological characters, at least in higher fungi.The bearing of our results and other known control mechanisms of the sexual cycle on the definition of the category 'species' are integrated in a proposed modification of the biological species concept: populations (races) belong to different species when the failure to interbreed and to produce viable offspring is not caused by genetic parameters operating in completion of the sexual cycle.