Two isolates of hyphomycetous fungus obtained from the root tissues of collapsed poplar trees formed white mycelia with pear-shaped swellings near septa in hyphae. Conidial state was observed in vitro in one-mutated isolate during the course of the study. Conidiophores are erect, brown, branched, more than 500μm tall, 2.5-2.8μm wide, bearing 2-30 conidia in two rows on the conidiogenous cells of the apical branches of the conidiophores. Conidia are hyaline or subhyaline, one-celled, elliptical, ovate or cylindrical, 3.7-5×2-2.2μm. After conidial detachment, crater-like scars were observed on the conidiogenous cells. The two original, and three-mutated isolates showed pathogenicity to Japanese black pine seedlings by the soil-over-agar-culture-inoculation method. On the basis of the morphological and pathological characteristics, the fungus was identified as Dematophora necatrix, the conidial state of white root rot pathogen, Rosellinia necatrix, although synnemata were not observed in vitro.