The aim of this study was to determine the effects of heat-killed Lactobacillus plantarum L-137 (HK L-137) on the growth performances and immune response of juvenile bighead catfish (Clarias macrocephalus). The experiment was conducted with four iso-nitrogenous (44.8 % crude protein) and iso-caloric (20.17 KJ g− 1 ) diets supplemented with different levels (0, 10, 20, and 50 mg kg− 1 ) of HK L-137. The bighead catfish (n = 1800, average body weight: 9.69 ± 0.24 g) were randomly introduced to 200-L composite tanks (filled with 150 L of fresh water) and stocked at 150 fish tank− 1 in triplicates per dietary treatment for 60 days. The results showed that the diets supplemented with 10, 20, and 50 mg kg− 1 of HK L-137 significantly increased the growth, protein efficiency ratio, feed conversion ratio, survival rate, and lysozyme activity compared to the control. Furthermore, the diets supplemented with 20 and 50 mg kg− 1 or 10 and 20 mg kg− 1 of HK L-137 significantly enhanced the total white blood cell count or apparent digestibility coefficient of the fish, respectively. The fish fed the HK L137 diets (at 10, 20, and 50 mg kg− 1 ) revealed lower cumulative mortality than the control after challenge with Aeromonas hydrophila. Our results suggested that dietary supplementation of 10 mg kg− 1 HK L-137 is the most efficient in improving the performance and immunity of bighead catfish; however, this should be confirmed in further study under full-scale commercial trials.