Abstract The effect of substituting Hermetia illucens (HI) meal for fish meal (FM) on the growth performance, survival rate, feed utilization, digestive enzyme activity, immunological function, and flesh quality of Clarias magur (average weight = 3.3 ± 0.05 g) is investigated in this work. Four iso-nitrogenous diets with 35% crude protein were formulated, each with varying levels of HI substitution: HI25%, HI50%, HI75%, and HI100%, then compared to a control diet (HI0%). Following a 90-day experiment, it was observed that the growth performance and feed efficiency were significantly improved at the 50% level (). However, the control group exhibited greater levels of digestive enzymes, specifically protease and lipase activity. The fish’s health status remained unaffected by the substitution of fishmeal, while the 50% HI group showed significant improvements. The high levels of the antioxidant enzymes SOD and CAT, as well as the metabolic product MDA, suggested enhanced antioxidant activity in the groups that received HI inclusion. However, as the substitution level reached 100%, the quality of the flesh decreased, as indicated by increased lipid and adhesiveness, as well as decreased L* and WI (). The findings of the study suggest that HI larvae can replace FM in diet of C. magur. Overall, at 50% level of HI larvae meal inclusion in diet, better growth and immune function was evident, but it does lead to a decrease in flesh quality. This study enhances the existing knowledge on substituting fish meal with insect meal in catfish that are of regional importance. Additionally, it emphasizes the necessity for additional investigation to examine the underlying mechanisms employing biotechnological approaches.