Bioactive substances such as β-carotene are regarded as an effective dietary aquafeed supplement to boost the immunity of fish cultured in high densities. The present study aimed to evaluate the influence of the dietary incorporation of β-carotene derived from Spirulina platensis (0, 0.5, and 1.0 g kg−1 diets) on growth performance, feed efficiency, hemato-biochemical indices, immunological responses, hepatic antioxidant enzymes, and associated immune-antioxidant genes expression in Nile tilapia, Oreochromis niloticus fingerlings (initial weight of 5.85 ± 0.06 g) reared at two stocking densities (SD100, 100 fish m−3 and SD200, 200 fish m−3) for 70 days. Fish were randomly allocated into eighteen plastic tanks (0.5 m3 each plastic tank) in triplicate and fed three times per day at 9:00 a.m., 11:00 a.m., and 3:00 p.m. with three isonitrogenous and isolipidic diets. The results showed that there was no …