Three experiments were conducted to evaluate the effects of dietary chitin and chitosan on growth, immune responses and resistance of grass prawn Penaeus monodon against white spot syndrome virus (WSSV) and Vibrio infections. In the first experiment, two levels (0.5¡B1 g/100g diet) of chitin and three levels (0.5¡B1¡B5 g/100g diet) of chitosan were evaluated. The results show that weight gain of the shrimp fed on diet containing no chitosan or the lowest level of chitosan (0.5 %) was higher than other groups. In the second experiment, four levels of chitosan (0¡B0.5¡B1¡B5 g/100g diet) were tested. Weight gains of the control (0 %) and 0.5 % chitosan groups were significantly (P<0.05) higher than the 0.1 and 1 % chitosan groups. Shrimp survival rate was not influenced by chitosan inclusion. The test shrimp of the first experiment were evaluated for their immune responses after dietary exposures. The results show that phenoloxidase activity and superoxide dismutase were not significantly different (P>0.05) among treatments. The production of superoxide anion in the 0.5 % chitin group was significantly (P<0.05) lower than the other groups at day 3 and 12. The last experiment evaluated the effectiveness of dietary chitosan against infection of WSSV and Vibrio damsela. Shrimp were fed for 20 days on test diets containing four levels of chitosan (0¡B0.5¡B1¡B5 g/100g diet) and then challenged by injection of WSSV or Vibrio solution. In the WSSV challenge, except at day 7, shrimp survivals were not different among treatments. At day 7, however, the survival rates of the shrimp fed the diet containing 0.1 or 1 % chitosan were significantly (P<0.05) higher than those of the other groups. When challenged with Vibrio damsela, there was no difference in shrimp survival among dietary treatments. The present study shows that dietary chitin and chitosan do not significantly enhance immune responses and disease resistance of juvenile P. monodon. Dietary incorporation of chitin or chitosan negatively affects shrimp growth.