The genome of Caldicellulosiruptor saccharolyticus encodes a range of glycoside hydrolases (GHs) that mediate plant biomass deconstruction by this bacterium. Two GH-based genomic loci that appear to be central to the hydrolysis of hemicellulosic and cellulosic substrates were examined. XynB-XynF (Csac_2404-Csac_2411) encodes intracellular and extracellular GHs that are active towards xylan and xylan side-chains, as well as carboxymethyl cellulose (CMC). XynD (Csac_2409) and XynE (Csac_2410) were produced recombinantly and confirmed to be xylanases. XynF (Csac_2411) was produced in two separate polypeptides, each with one GH43 catalytic domain displaying α-L-arabinofuranosidase activity. CelA-ManB (Csac_1076-Csac_1080) encodes four multi-domain, extracellular GHs, including CelB (Csac_1078), a 118 kDa extracellular enzyme not present in the other genome-sequenced member of this genus, Caldicellulosiruptor bescii (formerly Anaerocellum thermophilum). CelB contains both GH10 and GH5 domains, separated by a family 3 carbohydrate-binding module (CBM3). CelB encoded in Csac_1078 differed from the version originally reported (Saul et al., 1990, Appl Environ Microbiol 56:3117–3124) with respect to linker regions. CelB hydrolyzed xylan and CMC, as well as barley b-glucan, glucomannan, and arabinoxylan. For all substrates tested, intact CelB was significantly more active than either the individual GH5 and GH10 domains or the two discrete domains together, indicating that the multi-domain architecture is essential for complex carbohydrate hydrolysis. Transcriptomes for C. saccharolyticus grown at 70°C on glucose, xylose, xyloglucan, switchgrass, and poplar revealed that certain GHs were particularly responsive to growth on switchgrass and poplar and that CelB was in the top decile of all transcripts during growth on the plant biomass.