The gut microbiota of termites and cockroaches represents complex metabolic networks of many diverse microbial populations. The distinct microenvironmental conditions within the gut and possible interactions among the microorganisms make it essential to investigate how far the metabolic properties of pure cultures reflect their activities in their natural environment. We established the cockroach Shelfordella lateralis as a gnotobiotic model and inoculated germfree nymphs with two bacterial strains isolated from the guts of conventional cockroaches. Fluorescence microscopy revealed that both strains specifically colonized the germfree hindgut. In diassociated cockroaches, the facultatively anaerobic strain EbSL (a new species of Enterobacteriaceae) always outnumbered the obligately anaerobic strain FuSL (a close relative of Fusobacterium varium), irrespective of the sequence of inoculation, which showed that precolonization by facultatively anaerobic bacteria does not necessarily favor colonization by obligate anaerobes. Comparison of the fermentation products of the cultures formed in vitro with those accumulated in situ indicated that the gut environment strongly affected the metabolic activities of both strains. The pure cultures formed the typical products of mixed-acid or butyrate fermentation, whereas the guts of gnotobiotic cockroaches accumulated mostly lactate and acetate. Similar shifts toward more-oxidized products were observed when the pure cultures were exposed to oxygen, which corroborated the strong effects of oxygen on the metabolic fluxes previously observed in termite guts. Oxygen microsensor profiles of the guts of germfree, gnotobiotic, and conventional cockroaches indicated that both gut tissue and microbiota contribute to oxygen consumption and suggest that the oxygen status influences the colonization success. M any insects, particularly those feeding on a fiber-rich diet, possess a dense and complex microbiota. The most prominent examples are termites, whose ability to thrive on an entirely lignocellulosic diet depends on the digestive and nutritional contributions of microbial symbionts housed in their intestinal tracts (1, 2). During recent years, the microbial community structure of many termites has been studied in detail, and the evolutionary patterns in the gut microbiota of termites and their closest phylogenetic relatives, the cockroaches, are slowly emerging (3). In particular, the application of high-throughput sequencing techniques provides sufficient resolution and sampling depth to distinguish the phylogenetic and environmental drivers of the community structure (4,5,6).The functional roles of individual community members and their interactions, however, are more difficult to elucidate, mostly due to their formidable resistance to cultivation. Metagenomic and metatranscriptomic approaches have provided the first insights into the functional potentials of the gut community (7,8,9), but owing to the lack of reference genomes for many deepbranching lineages of the gut microbiot...