Abstract: The ability of two Fusarium species to produce beauvericin in the presence or absence of benomyl was investigated. Production of beauveracin was measured by determining its antagonistic activity on some selected pathogens using agar cup plate and agar plug plate techniques. The two methods were found to be reliable for the susceptibility test of Pseudomonas aeruginosa to the metabolite produced by Fusarium sp. The results of this research work showed that only in the absence of benomyl in most cases allowed Fusarium sp. to exact antagonistic potency on some pathogenic bacteria like P. aeruginosa, Staphylococcus aureus. Nevertheless, the benomyl concentration of 0.002 g mL-1could probably cause genetic modification to occur in F. solani that would stimulate the ability of the fungus to synthesize the antibiotic beauvericin that is capable of inhibiting Klebsiella pneumoniae. The presence of benomyl in the soil at high concentration of 0.8 g mL-1 was found to have adverse effects on the microbial population and interaction of the rhizosphere microorganisms of cowpea (Vigna unguiculata).