Technical Abstract: Antagonisms among microorganisms are strategies that maintain both inter- and intra- specific competition which is particularly important among those microorganisms that are ecological homologues, such as fungi and bacteria, that colonize the intercellular spaces of plants as endophytes. Recently endophytic microorganisms are being used as biocontrol agents, especially endophytic bacteria. A biocontrol bacterium, Bacillus mojavensis, was patented as a nonpathogenic biocontrol agent of plant diseases. However, before this bacterium can be used as a biocontrol agent, it must be evaluated against competing organisms, most are which are equally successful endophytes. This is the case for species of Fusarium fungi that are symptomless endophytes, especially on maize. During preliminary trails with this bacterium, it was discovered that Fusarium verticillioides (= F. moniliforme) was capable of colonizing the endophytic habit in spite of its being prior occupied by B. mojavensis. It was determined that fusaric acid produced by the fungus accounted for the reduction of bacterial colonizing and the resulting biocontrol activity. Fusaric acid at concentrations as low as 22 M accounted for a 41% reduction in control and that at 223 M and higher, there was a total kill of this bacterium. However, mutants of F. verticillioides were developed, which produced very low levels of fusaric acid, and were controlled by the bacterium in vitro and in planta. These studies indicate that fusaric acid is a self-defensive compound, use to control the growth of microorganisms. This substantiates an earlier report that fusaric acid controls biocontrol bacteria, but the first report indicating that it is toxic to a biocontrol bacterium, at least to B. mojavensis.