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In aquaculture, diseases and disease-outbreak are a major factor limiting its sustainability. Immunostimulants are an effective alternative to antibiotics, chemotherapeutics and perhaps even vaccines. Immunostimulants are a promising and safe prophylactic measure against infectious diseases. In this study, the immunostimulatory effect of the neutral water-soluble polysaccharide fraction (PF) isolated from the leaves of the hemi-parasitic mistletoe, Dendrophthoe falcata (DF) was assessed in Nile tilapia, Oreochromis niloticus. Fish weighing 40±5 g were injected intraperitoneally with Dendrophthoe falcata polysaccharide fraction (DFPF) at a dose of 2, 20 or 200 mgkg-1. An untreated control group and a positive control group (MacroGard™, 20 mgkg-1) were also maintained. The serum lysozyme, myeloperoxidase, antiprotease and bactericidal activities were significantly enhanced on post-treatment days when compared with that of the untreated control. In the challenge studies, after 7 and 21 days’ administration of DFPF, the high dose of 200 mgkg-1 caused a significant reduction in percent mortality against Aeromonas hydrophila infection. By semi-quantitative gene expression analysis, it was found that there was also a significant upregulation of IL-1β and lysozyme genes. After field trials, DFPF can be applied as an immunostimulant to finfish in aquaculture.