ABSTRACT The extremely thermophilic, Gram-positive bacteria Caldicellulosiruptor bescii and Caldicellulosiruptor obsidiansis efficiently degrade both cellulose and hemicellulose, which makes them relevant models for lignocellulosic biomass deconstruction to produce sustainable biofuels. To identify the shared and unique features of secreted cellulolytic apparatuses from C. bescii and C. obsidiansis , label-free quantitative proteomics was used to analyze protein abundance over the course of fermentative growth on crystalline cellulose. Both organisms' secretomes consisted of more than 400 proteins, of which the most abundant were multidomain glycosidases, extracellular solute-binding proteins, flagellin, putative pectate lyases, and uncharacterized proteins with predicted secretion signals. Among the identified proteins, 53 to 57 significantly changed in abundance during cellulose fermentation in favor of glycosidases and extracellular binding proteins. Mass spectrometric characterizations, together with cellulase activity measurements, revealed a substantial abundance increase of a few bifunctional multidomain glycosidases composed of glycosidase (GH) domain family 5, 9, 10, 44, or 48 and family 3 carbohydrate binding (CBM3) modules. In addition to their orthologous cellulases, the organisms expressed unique glycosidases with different domain organizations: C. obsidiansis expressed the COB47_1671 protein with GH10/5 domains, while C. bescii expressed the Athe_1857 (GH10/48) and Athe_1859 (GH5/44) proteins. Glycosidases containing CBM3 domains were selectively enriched via binding to amorphous cellulose. Preparations from both bacteria contained highly thermostable enzymes with optimal cellulase activities at 85°C and pH 5. The C. obsidiansis preparation, however, had higher cellulase specific activity and greater thermostability. The C. bescii culture produced more extracellular protein and additional SDS-PAGE bands that demonstrated glycosidase activity.