The medicinal leech has established a long-term mutualistic association with Aeromonas veronii, a versatile bacterium which can also display free-living waterborne and fish-or human-pathogenic lifestyles. Here, we investigated the role of antibiotics in the dynamics of interaction between the leech and its gut symbiont Aeromonas. By combining biochemical and molecular approaches, we isolated and identified for the first time the antimicrobial peptides (AMPs) produced by the leech digestive tract and by its symbiont Aeromonas. Immunohistochemistry data and PCR analyses evidenced that leech AMP genes are induced in the gut epithelial cells when Aeromonas load is low (starved animals), while repressed when Aeromonas abundance is the highest (post blood feeding). The asynchronous production of AMPs by both partners suggests that these antibiotic substances (i) provide them with reciprocal protection against invasive bacteria and (ii) contribute to the unusual simplicity of the gut microflora of the leech. This immune benefit substantially reinforces the evidence of an evolutionarily stable association between H. verbana and A. veronii. Altogether these data may provide insights into the processes making the association with an Aeromonas species in the digestive tract either deleterious or beneficial. Many, if not most, eukaryotic organisms have evolved intimate associations with symbiotic microorganisms 1. Mutualistic symbiotic interactions provide multiple advantages ranging from nutritional contribution to mediation of defense against pathogens and other natural enemies 2. An important question regarding such mutualistic interactions is to assess how such interactions can remain evolutionarily stable , with no partner evolving towards " cheater " strategies 3–5. Because of the potential for external bacteria to colonize the digestive tract, the study of gut microbiota symbiosis is particularly well suited to empirically assess the functional aspects of stable mutualistic interactions as well as the importance of vertical vs. horizontal modes of symbiont transmission. One way to tease apart these interspecific interactions is to use a biological model housing a simple microflora such as the medicinal leech Hirudo verbana. Indeed the gut of this hematophagous annelid is mostly dominated (> 95%) by two Gamma proteobacteria species, Aeromonas veronii and Mucinivorans