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Lactococcosis [Lactococcus garvieae (LG)] is one of the most prevalent bacterial diseases affecting grey mullet (Mugil cephalus) aquaculture. Therefore, the present research evaluated the efficacy of formalin‐killed LG vaccine with an oil‐based adjuvant in grey mullet under laboratory and field trials. The laboratory evaluation for LG vaccine and its cross‐protection upon challenge in grey mullet found that single‐dose immunization of formalin‐killed LG with adjuvant resulted in 91.4% and 100% relative per cent survival (RPS) when challenged with homologous and heterologous strains. The levels of specific antibody titre and lysozyme activity increased significantly in the vaccinated group. Immune gene expression at 24 hr after challenge showed an increase in levels of pro‐inflammatory and anti‐inflammatory cytokines. A parallel field trial experiment was conducted to investigate the long‐term effectiveness of the LG vaccine. Results demonstrated that at one month and three months post‐immunization with heterologous strain, 100% RPS was recorded in the vaccinated group. The findings suggested that the formalin‐inactivated LG vaccine strain (S3) protected grey mullet against LG infection for a period of three months. [ABSTRACT FROM AUTHOR]