Interactions between species shape the formation and function of microbial communities. In the gut microbiota of animals, cross-feeding of metabolites between microbes can enhance colonization and influence host physiology. We examined a mutually beneficial interaction between two bacteria isolated from the gut microbiota of , i.e., and After developing an coculture assay, we utilized a genetic screen to identify genes required for enhanced growth with The screen, and subsequent genetic analyses, showed that the gene encoding pyruvate phosphate dikinase () is required for to benefit fully from coculture. By testing strains with mutations in a range of metabolic genes, we provide evidence that can utilize multiple fermentation products of Mutualism between the bacteria affects gnotobiotic ; flies associated with and showed >1,000-fold increases in bacterial cell density and significantly lower triglyceride storage than monocolonized flies. Mutation of decreased density in flies cocolonized with , consistent with the model in which employs gluconeogenesis to assimilate fermentation products as a source of carbon We propose that cross-feeding between these groups is a common feature of microbiota in The digestive tracts of animals are home to a community of microorganisms, the gut microbiota, which affects the growth, development, and health of the host. Interactions among microbes in this inner ecosystem can influence which species colonize the gut and can lead to changes in host physiology. We investigated a mutually beneficial interaction between two bacterial species from the gut microbiota of fruit flies. By coculturing the bacteria , we were able to identify a metabolic gene required for the bacteria to grow better together than they do separately. Our data suggest that one species consumes the waste products of the other, leading to greater productivity of the microbial community and modifying the nutrients available to the host. This study provides a starting point for investigating how these and other bacteria mutually benefit by sharing metabolites and for determining the impact of mutualism on host health.