Bacteroides thetaiotaomicron is a prominent member of the human distal gut microbiota that specializes in breaking down diet and host-derived polysaccharides. While polysaccharide utilization has been well studied in B. thetaiotaomicron, other aspects of its behavior are less well characterized, including the factors that allow it to maintain itself in the gut. Biofilm formation may be a mechanism for bacterial retention in the gut. Therefore, we used custom GeneChips to compare the transcriptomes of biofilm and planktonic B. thetaiotaomicron during growth in mono-colonized chemostats. We identified 1,154 genes with a fold-change greater than 2, with confidence greater than or equal to 95%. Among the prominent changes observed in biofilm populations were: (i) greater expression of genes in polysaccharide utilization loci that are involved in foraging of O-glycans normally found in the gut mucosa; and (ii) regulated expression of capsular polysaccharide biosynthesis loci. Hierarchical clustering of the data with different datasets, which were obtained during growth under a range of conditions in minimal media and in intestinal tracts of gnotobiotic mice, revealed that within this group of differentially expressed genes, biofilm communities were more similar to the in vivo samples than to planktonic cells and exhibited features of substrate limitation. The current study also validates the use of chemostats as an in vitro "gnotobiotic" model to study gene expression of attached populations of this bacterium. This is important to gut microbiota research, because bacterial attachment and the consequences of disruptions in attachment are difficult to study in vivo.