A functional bacterial consortium that can effectively hydrolyze cellobiose and produce bio-hydrogen was isolated by a concentration-to-extinction approach. The sludge from a cattle feedlot manure composting plant was incubated with 2.5-20 g l(-1) cellobiose at 35 degrees C and pH 6.0. The microbial diversity of serially concentrated suspensions significantly decreased following increasing cellobiose concentration, finally leaving only two viable strains, Clostridium butyricum strain W4 and Enterococcus saccharolyticus strain. This consortium has a maximum specific hydrogen production rate of 2.19 mol H(2)molhexose(-1) at 5 g l(-1) cellobiose. The metabolic pathways shifted from ethanol-type to acetate-butyrate type as cellobiose concentration increased from 2.5 to >7 g l(-1). The concentration-to-extinction approach is effective for isolating functional consortium from natural microflora. In this case the functional strains of interest are more tolerant to the increased loadings of substrates than the non-functional strains.