The biological control of rice diseases with antagonistic microbial organisms has been studied extensively and is considered a promising technology (Mew et al 1993). Many factors have been shown to affect the efficacy of biocontrol agents (BCAs), but there have been few studies on effects of pesticides on disease biocontrol (Zhao et al 1992). Some antibiotics have been reported to be effective against bacterial diseases of crops. It is important to determine whether antibiotic applications in the field affect disease suppression by BCAs. In biocontrol studies, mutation with antibiotics has been exploited to obtain bacterial mutants with acquired or increased antagonism to pathogens (Georgakopoulos et al 1994). Little information has been available on bacterial BCA mutations in relation to biocontrol of diseases in rice systems. The objectives of this study were to (1) analyze the direct effects of different antibiotics on the growth of BCAs, (2) obtain BCA mutants with enhanced biocontrol activity, and (3) examine the effects of antibiotics on rice sheath blight and BCA function.