Recent findings support the concept that Clostridium thermocellum is a cellulose-utilizing specialist having growth benefits with increasing substrate chain length. We developed a continuous-flow system for in-situ detection of cellulose colonization and qualitatively assayed metabolic activities and behaviour of cellulolytic cultures. This study demonstrates the existence of strongly adherent celluloytic cells arranged in monolayers with invariably end-on attached spores. The substrate-cell distance was recorded to be lower than 0.44 pm and a typical EPS matrix was absent. Measurements on carbon dioxide released in continuous-flow cultures was successfully employed to monitor biofilm activity and total carbohydrate assays do not reveal loss of cellulolysis end-products in the effluent. These findings demonstrate the bacteria have optimized access to the cellulosic substrates and suggest that they have an ability to sequester products of substrate hydrolysis which confers benefits over non-adherent cellulolytic or non-cellulolytic organisms.