Abstract: Largemouth bass (Micropterus salmoides), commonly known as California bass, has been introduced to China since the 1980s. After years of breeding development, it has now become one of the important species of freshwater aquaculture in China. However, with the rapid development of intensive aquaculture and irregular feeding methods, California bass diseases frequently occur. The traditional method to prevent and treat bacterial diseases in fish is to add antibiotics to aquafeeds. However, the abuse of antibiotics can lead to problems such as bacterial resistance, drug residues and environmental pollution. Therefore, the selection of suitable antibiotic substitutes for the aquaculture industry is an urgent matter, and it has far-reaching significance for the growing largemouth bass aquaculture industry. Research on alternatives to antibiotics has found that antimicrobial peptides are a kind of polypeptides produced by the non-specific immune system in organisms. Because of their unique biological activity, antibacterial and bactericidal mechanisms different from traditional antibiotics, they are not easy to produce drug resistance and no resistance. Pollution and other advantages are expected to be developed into a new type of high-efficiency antibacterial drugs, which have great potential to replace antibiotics in the field of feed additives. The bovine lactoferricin (LfcinB) used in this test is a cationic antimicrobial peptide containing 25 amino acid residues produced by pepsin hydrolysis of bovine lactoferrin under acidic conditions. It has a broad-spectrum and high-efficiency antibacterial ability and antiviral ability. However, its research as a feed additive has not been reported yet. This study investigated the effect of the dietary supplementation of bovine lactoferricin on growth performance, digestive enzymes activity, intestinal tissue structure and disease resistance against Aeromonas hydrophila in juvenile largemouth bass, aiming to evaluate its potential to replace antibiotics and provide a theoretical basis with the application of functional feed. A total of 450 tails largemouth bass with an average body weight of (19.88±0.03) g were randomly allocated into 5 groups with 3 replicates per group, and fed with basal diet (negative control), basal diet supplemented with 30 mg/kg florfenicol (positive control), basal diet supplemented with 1000, 1500, and 2000 mg/kg bovine lactoferricin, respectively. The experiment lasted for 8 weeks. The results showed that: (1) With the increase of the addition of bovine lactoferricin, final body weight (FBW), weight gain rate (WGR) and specific growth rate (SGR) of largemouth bass showed a trend of first increasing and then decreasing. The 1000 mg/kg bovine lactoferricin group had the best growth performance of largemouth bass, which was significantly different from the negative and positive control groups (P<0.05). (2) Compared with the negative and positive control groups, the intestinal trypsin, α-amylase and lipase activities of the 1000 mg/kg bovine lactoferricin group of largemouth bass increased significantly (P<0.05). (3) Compared with the negative and positive control groups, the addition of 1000 mg/kg bovine lactoferricin to feed could significantly increase the height and width of villi of the foregut, midgut and hindgut of largemouth bass (P<0.05). (4) After challenge with Aeromonas hydrophila, the survival rates of largemouth bass in the bovine lactoferricin groups were higher than that of the negative control group, but there were no significant differences from the positive control group (P>0.05). In summary, under the experimental conditions, the addition of bovine lactoferricin to feed could improve growth performance of juvenile largemouth bass, and the appropriate dosage was 1000 mg/kg. At the same time, bovine lactoferricin could increase the intestinal digestive enzyme activity of largemouth bass, improve intestinal tissue structure, and improve disease resistance.