The Fusarium spp. mycotoxins fusaric acid and deoxynivalenol (DON) were tested for antimicrobial activity against Ruminococcus albus and Methanobrevibacter ruminantium. The growth of both organisms was inhibited by fusaric acid as low as 15 micrograms/mL (84 microM) but not by DON, at levels as high as 100 micrograms/mL (338 microM). No synergistic inhibitory effect was observed with DON plus fusaric acid. Neither organism was able to adapt to the fusaric acid and responses of each organism to the compound were different. The optical density (OD) maximum for R. albus, but not for M. ruminantium, was diminished after 28 days incubation at concentrations of fusaric acid below 240 micrograms/mL. Inhibition of R. albus started before significant growth had occurred, while M. ruminantium doubled twice before the onset of inhibition. Responses to picolinic acid, an analog of fusaric acid, were also dramatically different between the two microorganisms with M. ruminantium exhibiting a severe lag followed by a complete recovery of growth, while R. albus was only slightly inhibited with no lag. These results suggest that the mechanism of fusaric acid inhibition is specific to each microorganism. This is the first demonstration of the common mycotoxin fusaric acid inhibiting the growth of rumen bacteria.