Abstract: Indole signaling has been regarded as a promising target to control aquatic diseases. However, the relationship between exogenous indole and the virulence of Edwardsiella piscicida is obscure. E. piscicida is a facultative intracellular pathogen, and has been a model strain in aquaculture. In this study, we investigated the effect of exogenous indole on stress resistance and virulence of E. piscicida in the presence of and absence of endogenous indole (WT and ΔtnaA, respectively). Our results showed that exogenous indole reduced the resistance of WT against strong acidic stress, but enhanced that of ΔtnaA. Also, we found that exogenous indole abated viability of E. piscicida at high temperature, repressed bacterial biofilm formation, impaired bacterial envelope integrity, and weakened bacterial proliferation in macrophages, irrespective of the presence or absence of endogenous indole. These virulence-related phenotypes caused by exogenous indole are reasonably explained by the observation that exogenous indole downregulated the expressions of CpxRA and its target YccA via being responded by CpxA. The effects of exogenous indole on strong acid resistance are partially achieved by changing the expression of GadD, the key functional enzyme of acid resistance system (AR2). We believe that this is the first report about the impact of exogenous indole on strong acid stress and membrane integrity of pathogenic bacteria. Also, we reveal the likely mechanism by which exogenous indole regulates the expressions of virulence-related genes. These findings provide a new understanding on pathogenesis of E. piscicida and contribute to the prevention and control strategies of edwardsiellosis.