BACKGROUND: Fish gills serve as critical immune interfaces against aquatic pathogens, yet their leukocyte heterogeneity in response to parasitic infections remains poorly understood. METHODS: Single-cell RNA sequencing was employed to elucidate leukocyte responses in the gills of during infection. RESULTS: A total of 13,070 leukocytes from the gills under steady-state and infected conditions were profiled and classified into eight principal lineages: T cells (> 70% of total immune cells), ILC2-like cells, NK-like cells, neutrophils, granulocytes, B cells, macrophages, and dendritic cells. Following infection, T cell subsets exhibited distinct responses: Regulatory T cells expanded and demonstrated immunoregulatory capacity; CD8 T cells exhibited cytotoxic responses; CD4CD8 T cells displayed Th17-like functions; and γδ T cells showed Th2-like activity. ILC2-like cells significantly increased in abundance and upregulated type 2 cytokine expression, whereas cytotoxic NK-like cells enhanced chemokine signaling and cytotoxicity. Neutrophils increased in number and oxidative activity, while granulocytes highlighted immunomodulatory functions. Macrophages, dendritic cells, and B cells exhibited compartmentalized activation states, upregulating gene modules associated with pathogen recognition, antigen processing/presentation, chemotactic activity, and antibody defenses. CONCLUSIONS: These findings describe a multi-layered immune cell defense strategy in the gills of teleosts against parasitic infection, showing conserved and fish-specific adaptations. Understanding gill immunity provides viable targets for enhancing parasite resistance in aquaculture, such as modulating ILC2/Treg pathways to prevent infections.