Technical Abstract: In an attempt to develop attenuated bacteria as potential live vaccines, four chemicals (gossypol, proflavine hemisulfate, novobiocin, and ciprofloxacin) were used to modify the following four genera of bacteria through chemical-resistance strategy: 1) Aeromonas hydrophila (9 isolates); 2) Edwardsiella tarda (9 isolates); 3) Streptococcus iniae (9 isolates); and 4) S. agalactiae (11 isolates). All bacteria used in this study were able to develop high resistance to gossypol. However, only some bacteria were able to develop resistance to proflavine hemisulfate, novobiocin, or ciprofloxacin. When the virulence of resistant bacteria was tested in tilapia or catfish, none of the gossypol-resistant isolate was attenuated, whereas majority of the proflavine hemisulfate-resistant isolates were attenuated. However, all proflavine hemisulfate-attenuated bacteria failed to provide significant protection to fish. Eight novobiocin- or ciprofloxacin- resistant Gram-positive bacteria (S. agalactiae and S. iniae) were found to be attenuated. However, none of them offered protection higher than 70%. Of seven attenuated novobiocin- or ciprofloxacin- resistant Gram-negative isolates (A. hydrophila and E. tarda), only one (novobiocin-resistant E. tarda 30305) was found to relatively safe and highly efficacious. When E. tarda 30305-novo vaccinated Nile tilapia were challenged by its virulent E. tarda 30305, relative percent of survival of vaccinated fish at 14- and 28- days post vaccination (dpv) was 100% and 92%, respectively. Similarly, E. tarda 30305-novo offered 100% protection to channel catfish against challenges with virulent parent isolate E. tarda 30305 at both 14- and 28- dpv. Our results suggest that the development of live attenuated bacterial vaccines that are safe and efficacious is challenging, although it is feasible.