Geotrichum candidum is a filamentous fungus mostly used in the dairy industry for cheese ripening and flavoring. The current study was designed to evaluate the potential probiotics effect of dietary supplementation of G. candidum on growth, digestive enzymes activities, gut microbiota and immune-related gene expression and disease resistance in Gibel carp CAS III (Carassius auratus gibelio) against A. hydrophila infections. For this purpose, 420 healthy fish (initial body weight: 7.09 +/- 0.02g) were randomly divided into 7 feeding groups in triplicates of 20 fish in each tank. Seven isocaloric and isonitrogenous diets were prepared, supplemented with 6 different doses of G. candidum viz; C: 0, T1:10(6), T2:10(7), T3:10(8), T4:10(9), T5:10(10), T6:10(11) CFU/kg diet and fed to fish twice a day for 60 days. Results showed significantly higher weight gain (WG) and feed efficiency (FE) in groups fed 10(6) and 10(8) CFU/kg diet compared to the control (P < 0.05). All G. candidum fed groups showed higher intestinal a-amylase activity (P < 0.05) while no difference in lipase and trypsin activity compared to the control group. Before challenge test, gut microbial diversity analysis revealed relatively more abundance of bacteria belonging to phylum Proteobacteria, Actinobacteria, Firmicutes and Bacteriodetes in group fed G. candidum supplemented diet and higher percentage of pathogenic bacteria belonging to order Aeromonadales and Vibrionales in control post-challenge. After challenge test immune response expressed numerically higher survival rate (P > 0.05) and significantly higher (P < 0.05) Respiratory Burst Activity(RBA), Immunoglobulin M level (IgM), Aspartate Transaminase activity (AST) and improved (P > 0.05) lysozyme activity, and Alanine Transaminase activity (ALT) in groups fed 10(6) and 10(8) CFU/kg G. candidum supplemented diet. The expression of immune related gene (IL-I beta, TNF- alpha, HSP70 and TLR-2) in liver of the fish were significantly affected (P < 0.05) by supplementation of G. candidum post bacterial challenge. The results showed that G. candidum is a potential probiotic as it could improve feed utilization, immunity and cause no harmful effects on growth of gibel carp at higher supplementation levels. The productive results can be achieved by using the lower dose as 10(6)-10(8) CFU/kg diet.