Aims: The goals of this study were to determine if beta-acid from hops (Humulus lupulus L.) could be used to control fructan fermentation by equine hindgut micro-organisms, and to verify the antimicrobial mode of action on Streptococcus bovis, which has been implicated in fructan fermentation, hindgut acidosis and pasture-associated laminitis (PAL) in the horse. Methods and Results: Suspensions of uncultivated equine faecal microorganisms produced fermentation acids when inulin (model fructan) was the substrate, but beta-acid (i.e. lupulone) concentrations >= 9 ppm inhibited lactate production and mitigated the decrease in pH. Inulin-fermenting Strep. bovis was isolated from the beta-acid-free suspensions after enrichment with inulin. The isolates were sensitive to beta-acid, which decreased the viable number of streptococci in faecal suspensions, as well as growth, lactate production and the intracellular potassium of Strep. bovis in pure culture. Conclusions: These results are consistent with the hypothesis that hops beta-acid prevented the growth of fructan-fermenting equine faecal bacteria, and that the mechanism of action was dissipation of the intracellular potassium of Strep. bovis. Significance and Impact of the Study: Bacterial hindgut fermentation of grass fructans has been linked to PAL and other metabolic disorders in horses. Hops beta-acid is a potential phytochemical intervention to decrease the growth of bacteria responsible for PAL.