A new icosapeptide, aibellin, markedly modified rumen fermentation in vitro. Batch culture experiments with mixed rumen microorganisms showed that 12.5 to 25 mg/L of aibellin enhanced propionate production and reduced methanogenesis without significantly affecting production of total VFA, protozoal survival, or cellulose digestion. Aibellin had essentially the same effects in continuous culture with hay powder and concentrate. Monensin (5 mg/L) had similar effects on propionate production and methanogenesis, but total VFA, protozoa, and cellulolysis were decreased even by this low concentration of monensin. Commercially available peptide antibiotics also were compared with aibellin. Of the antibiotics examined, only graminicidin D (7.5 to 15 mg/L) enhanced propionate production and reduced methanogenesis. However, gramicidin D decreased total VFA, protozoa, and cellulolysis even at 7.5 mg/L. Alamethicin (7.5 to 15 mg/L), which resembles aibellin in its structure, did not increase propionate production but raised the percentage of propionate because of reduced production of total VFA. Alamethicin depressed methanogenesis but also decreased protozoal survival and cellulose digestion. These in vitro experiments indicate that aibellin could be a useful and potent modifier of rumen fermentation.