Aims: To evaluate the feasibility of using an in vitro cell assay to select attenuated bacterial mutants. Methods and Results: Using catfish gill cells G1B, the feasibility of using an in vitro assay instead of in vivo virulence assay using live fish to select attenuated bacterial mutants was evaluated in this study. Pearson correlation analysis between in vitro virulence to G1B cells and in vivo virulence of Aeromonas hydrophila and Edwardsiella tarda revealed that there was a significant correlation between the two (r = -0.768, P value = 3.7 x 10(-16)). Conclusions: The in vitro cell assay might be initially used to screen large quantities of bacteria to select attenuated mutants of catfish pathogens. Significance and Impact of the Study: The in vitro cell assay using catfish gill cells to identify attenuated mutants of catfish pathogens will reduce cost involved in the in vivo virulence assay that requires many fish and aquariums.