The ultrastructure of conidia produced by the mycelia of Polyporus umbellatus was studied by use of scanning and transmission electron microscopy. The results suggested that only clamped (dikaryotic) mycelia produced aerial conidiophores with ovoid to rod-shaped conidia. There was a distinctive bulge near the top of the conidium. The contents of the conidia were dense. This type of conidium could be regarded as arthroconidia. The ultrastructure of conidia produced by the mycelia of Polyporus umbellatus was studied by use of scanning and transmission electron microscopy. The results suggested that only clamped (dikaryotic) mycelia produced aerial conidiophores with ovoid to rod-shaped conidia. There was a distinctive bulge near the this of the conidium could be as as arthroconidia.