The aim of this experiment was to determine the effects of dietary inclusion with mannan oligosaccharide (Bio-Mos, Alltech, Nicholasville, KY, USA) on growth, survival, physiological and immunological conditions and gut morphology of the black tiger prawn (Penaeus monodon). Five diets supplemented with MOS at 0 g kg−1 (control diet), 1, 2, 4 and 8 g kg−1 were fed to the prawn juveniles (0.4 ± 0.06 g, total weight) for the duration of 63 days. Growth was the highest (P < 0.05) when the prawns were fed the 1 g kg−1 MOS included diet. Wet tail muscle index (Tw/B), dry tail muscle index (Td/B) and tail muscle protein (Tmp) were higher (P < 0.05) in the prawns fed MOS included diets MOS compared with the prawns fed the control diet. Total haemocyte counts (THCs) of the prawns fed MOS included diets were higher (P < 0.05) than THCs of the prawns fed the control diet. Epithelium layer and epidermal cell density of the gut of the prawns fed 1 g kg−1 and 2 g kg−1 MOS diets were better than the prawns fed the control and other MOS diets. The results imply a positive effect of dietary supplementation of 1–2 g kg−1 MOS in the culture of black tiger prawns.