The formation of xylanolytic enzymes by Butyrivibrio fibrisolvens NCFB 2249 was induced by xylan, xylo-oligosaccharides, and xylobiose. Inhibition of RNA or protein synthesis prevented induction, and enzyme formation occurred only when anaerobiosis was maintained. The rate of enzyme induction by xylan was affected by pH and inducer concentration, and highest levels of activity occurred when the initial pH and xylan concentration were pH 6.5-7 and greater-than-or-equal-to 2 mg/ml respectively. The ability of the cells to respond to the inducer was reduced in slowly growing cells, although cells that were grown at dilution rates that appertain in the rumen ecosystem responded rapidly to the inducer. Butyrivibrio fibrisolvens also exhibited diauxic characteristics of carbohydrate utilization, and in consequence enzyme induction and xylanolysis were delayed until readily metabolized sugars (e.g., glucose, arabinose) had been consumed.