During an investigation of mycoparasitic fungi on sclerotia of Sclerotinia sclerotiorum in China, a new fungal species was consistently encountered and isolated from natural soils taken from soybean fields of Shandong and Jiangsu Provinces. The fungus is featured by its sphaeroid conidia with 1-2 transverse septa, but mostly (>65%) with only one septum at the base. It resembles Monacrosporium indicum, M. sphaeroides and M. sinense, but can be distinguished from the first two species by lack of basal hila and large vacuoles on its conidia, respectively, and from M. sinense by its typically two-celled and broadly turbinate to napiform conidia. Colonization frequencies on S. sclerotiorum sclerotia by the new species were 10% and 33.3% in the two field soils, respectively, when the sclerotia were introduced into soils and coincubated at 22-24 degrees C for 4 wk. Reinoculation tests by placing surface-sterilized sclerotia onto the tested isolate colony for 2 wk and then surface-sterilized again resulted in 23.3% sclerotia colonized. Microscopic observations indicated that the fungus coiled around hyphae of Rhizoctonia solani and grew along and appressed to hyphae of Fusarium solani f.sp. pisi, S. sclerotiorum and Phytophthora cactorum when dual-cultured in slides. Tests on agar plates demonstrated that the fungus formed adhesive networks and was an active predator of Panagrellus redivivus. This study indicated the diverse mechanisms for the fungus to survive in soil. For expression of its mycoparasitic and nematode-trapping capacities, the fungus is named as Monacrosporium janus.