Using light and electron microscopy,Gliocladium virensis proved to be a hyperparasiteof Rhizoctonia solani. When hyphae ofG.virenscome in contact with those ofR.solani, the following sequence of events are observed (i) contact ofG.virenswith host cells, (ii) formation of appresoria, (iii) penetration of host cells, (iv) formation of intracellular hyphae, and (v) collapse and death of host cells.In vitro,G.virenseffectively inhibited sclerotial formation ofR.solani.Greenhouse tests showed that the presence ofG.virensin soil artificially infested withR.solanireduced at planting the severity ofRhizoctoniaroot rot in white beans. Root rot severity decreased with increasing concentrations ofG.virens. A similar result was obtained in soil treated with the two fungi 2 months prior to planting. This study suggests thatG.virensmay be a promising biological control agent forRhizoctoniaroot rot of white beans.