ABSTRACTFungitoxic effects of the fungusBalansia cyperiinfecting the sedgeCyperns rotunduswere examined. Comparisons of fungal colonizers ofB. cyperi-infected and uninfectedC. rotundusshowedRhizoctonia solanito inhabit leaves from uninfected plants but not leaves fromB. cyperi-infectedplants.In vitrobioassays with solvent extracts ofB. cyperimycelia and culture filtrates showed growth inhibition of test fungi, includingFusarium oxysporumandR. solani.Solvent extracts of leaves fromB. cyperi-infected plants inhibited growth of most test fungi includingF. oxysporum, R. oryzae, andR. solani.Extracts from uninfected plants inhibited the latter two species but to a lesser degree. Results suggest thatB. cyperiinfecting C.rotunduspotentially may deter infection of its host by other more pathogenic fungi.