Sclerotia ofSclerotinia sclerotiorumin soil were invaded by a destructive fungus that was isolated and identified asSporidesmium sclerotivorum. The parasitic fungus grew slowly on an autoclaved agar medium made from comminuted sclerotia ofS.sclerotiorumand on cornmeal agar but failed to germinate or grow on many common mycological media. Macroconidia ofS.sclerotivorumgerminated adjacent to sclerotia on water agar or moist filter paper, colonized the sclerotia, and developed two distinctive asexual spore states by which it could be recognized. In moist sand, steamed soil, and natural soil the mycoparasite infected and destroyed more than 95% of the amended (1% w/w) sclerotia ofS.sclerotiorumin 10 weeks or less at 25 °C. Sclerotia killed by autoclaving were poorly colonized. Sclerotia ofSclerotium cepivorumwere attacked by the mycoparasite more slowly than those ofS.sclerotiorum. Sclerotia ofMacrophomina phaseolinaapparently were not parasitized. The mycoparasite was isolated from soils from three different areas of the northeastern United States. The prolific development of the mycoparasite in soil containing sclerotia of susceptible species and its ability to spread through soil by growth from one sclerotium to another suggest thatS.sclerotivorumis an important contributor to the natural destruction of sclerotia in.soil and that it may have high potential as an applied agent of biological control of sclerotial fungi.