Paenibacillus polymyxa, a Gram-positive low-G+C spore-forming soil bacterium, belongs to the plant growth-promoting rhizobacteria. The swarming motility of P. polymyxa strain E681 was greatly induced by a secondary metabolite, citrinin, produced by Penicillium citrinum KCTC6549 in a dose-dependent manner at concentrations of 2.5-15.0 microg mL(-1) on tryptic soy agar plates containing 1.0% (w/v) agar. Flagellum staining showed that citrinin activated the production of flagella by P. polymyxa. This result was supported by reverse transcriptase-PCR analysis of gene expression, which showed increased transcriptional levels of sigD and hag homologues of P. polymyxa E681 in the presence of citrinin. The results presented here show that a mycotoxin, citrinin, has a newly identified function of inducing bacterial motility by transcriptional activation of related genes. This finding contributes to our understanding of the interactions between bacteria and fungal strains in nature.