Four mating types of basidispores were determined by using three-cycle mating system, three strains of Panus giganteus were used as test materials. Microscopical examination revealed that basidia of P. giganteus produced four basidiospores. The results of mating tests showed that P. giganteus has a tetrapolar heterothallism mating system. χ2 tests indicated that the distribution of the four mating types among spore monokaryons was in accordance with the 1:1:1:1 law of segregation in two strains, but one strain did not display the expected segregation ratio of basidiospores.