Bacterial aro mutants are frequently used as live attenuated vaccines for domestic animals. In this study, we characterized Edwardsiella tarda strain LSE40 with a deletion in the aroA gene. In addition to autotrophy, the aroA mutant appeared to have delayed cell division and reductions in its swarming motility, biofilm formation, and production of translocator proteins in the type III secretory system. The mutant exhibited high virulence attenuation in turbot fish, Scophthalmus maximus (L.), where the 50 % lethal dose increased by more than 3 log(10) via intraperitoneal (i.p.) injection and by > 2 log(10) via immersion exposure compared with the wild-type parent strain. A tissue persistence study showed that the mutant retained the ability to invade and spread in turbot and viable cells could be detected up to 28 days after i.p. infection and 21 days after immersion exposure. These results suggested a pleiotropic role for aroA in the physiological behavior of E. tarda. Turbot exhibited a good humoral response and the enhanced expression of innate immune factors, interleukin 1 beta and lysozyme, when vaccinated with aroA mutant at 10(5) CFU via i.p. injection and at 10(8) CFU via immersion exposure. However, the aroA mutant did not provide effective protection for turbot against edwardsiellosis following i.p. vaccination at doses of 10(4)-10(6) CFU or immersion vaccination at doses of 10(6)-10(8) CFU ml(-1). We hypothesized that the aroA mutant did not trigger an appropriate T cell-immune response in turbot against infection of E. tarda.