Accumulating evidence suggests that the gut microbiota influences host neurological activity and behaviour. Here, the authors find that the two gut Firmicutes, Clostridium sporogenes and Ruminococcus gnavus, produce and secrete two distinct tryptophan decarboxylases that catalyse the formation of the neurotransmitter tryptamine. Tryptamine induces the release of serotonin, which is suggested to modulate gastrointestinal motility; consistent with this, the authors show that tryptamine affects colonic ion secretion. In addition, a reduction in plasma tryptophan levels owing to the activity of these enzymes would reduce serotonin levels in the brain, which suggests a possible direct mechanism by which the gut microbiota influences host behaviour. Although this enzymatic activity is rare in bacteria in general, analysis of data from the Human Microbiome Project reveals that at least 10% of the human population contains a gut bacterium that encodes a tryptophan decarboxylase.