When the mycelial mat of Trichoderma reesei QM 6a was treated with the mitotic arrester colchicine for 30 days, various sizes of autopolyploid nuclei were produced in the mycelial mat. Such mycelia were named multinuclear autopolyploids. As the conidium of this fungus is mononucleate, only an autopolyploid nucleus was present in each conidium. Thus, it was concluded that genetically stable autopolyploid strains can be selected easily from among the conidia in this fungus.