Abstract: To investigate the distinctions between the highly virulent Nocardia seriolae strain LY20810 F1 and its counterpart LY20810 F110, induced through prolonged passages, a comparative analysis was conducted encompassing biological traits, virulence gene sequences, pathogenicity, and induced host immune responses. Furthermore, the immunoprotective effects of the attenuated strain LY20810 F110 on largemouth bass were evaluated. The results showed significant discrepancies in colony morphology, cell morphology, and growth rate between the attenuated strain LY20810 F110, induced through 110 continuous passages in BHI medium, and the wild-type strain LY20810 F1. Analysis of 10 virulence gene sequences demonstrated single non-synonymous mutations in genes SodC and ESAT6 of strain LY20810 F110. Pathogenicity testing exhibited that while the highly virulent strain LY20810 F1 induced 100% mortality in largemouth bass within 30d at a dose of ×10⁷ cfu/mL, the attenuated strain LY20810 F110 did not cause fish mortality at the same infection dose. Upon infection, strain LY20810 F1 prompted significant upregulation of inflammatory cytokines, including IL-1β, IL-10, TNF-α, and IFN-γ in the spleen and head kidneys of largemouth bass after 3d, indicating an inflammatory response. Conversely, strain LY20810 F110 induced upregulation of eight immune factors, such as IgM, CD4-α, CD8-α, MHCI-α at 14d and 28d. Immune protection tests demonstrated the efficacy of both injection and bath vaccination with strain LY20810 F110 (1×10⁸ cfu/mL) in safeguarding largemouth bass spleen and head kidneys from pathological damage caused by N. seriolae infection. Relative protection rates for largemouth bass were 75% and 66.7% for injection and bath vaccination, respectively. The above findings indicated that the attenuated strain LY20810 F110holds promise as a candidates for live vaccine development to control fish nocardiosis in aquaculture. The study provides a scientific foundation for the development of attenuated Nocardia vaccines.