The present study was conducted to investigate the behavioral, biochemical, and molecular responses of Oreochromis niloticus fish exposed to sublethal concentration of thallium (T1; 1/10th of 96-h LC50 =41.9 μg/L) for 60 days. The beneficial role of Astragalus membranaceus polysaccharides (ASP) as a dietary supplement (0.15% and 0.30% diet) in attenuating the resulted alterations was also studied. T1 increased the agressive behaviuor of fish in asociasion with reduction of brain levels of serotonine, GABA and amino acids neurotransmitters. Moreover, the activities of Na+/K+-ATPase and acetylcholine esterases (AchE) were reduced following exposure to T1. T1 also diminsh the catalase (CAT) activity in the brain while elevated the content of oxidative stress markers (protein carbonyl; PC and malondialdhyde; MDA) and DNA damage variables together with overexpressions of the TNF-α, IFN-γ, and IL-1β and stress related genes (P53, Caspase-3 and HSP70) in the brain of fish. In addition, T1 exposure upregulated the expressions of stress related genes, P53, Caspase-3 and HSP70 in the brain. Co-supplementation of ASP (especially at 0.30% diet) with T1 significantly ameliorated the toxic effects of the trace metal. The observasions of the present study indicated the neuroprotective activity of ASP represented by antioxidant, antiapoptotic and anti-inflammatory events evoked by ASP in T1-exposed fish. In conclusion, the supplementation of O. niloticus diet with ASP might be a promising step for reducing the stress of pollution and keeping the behavior pattern and health of the fish.