Study on natural immune responses of freshwater prawn, Macrobrachium rosenbergii (De Man 1879) exposed to deltamethrin in laboratory condition was performed. Prawns (17±2.3 g) were maintained in water containing concentrations of deltamethrin at 0.0025; 0.025 and 0.125 mg.L-1.  Prawns in control group were maintained in water without deltamethrin. Haemolymph samples from prawns in all groups were collected at 0, 24, 48, 72 and 96 h after exposure to deltamethrin. Haemolymph analyses carried out include total haemocyte count and identification, phenoloxidase, respiratory burst and superoxide dismutase activities. Results of immunological analyses revealed that total haemocyte count, granular cells, hyaline cells and phenoloxydase activity decreased significantly (Pexposure. Respiratory burst activity in the treatment groups increased significantly (PHowever, in the group exposed to 0.125 mg.L-1 deltamethrin, superoxide dismutase activity decreased at 24, 72 and 96 h exposure to deltamethrin compared to the control group, but no difference was observed at 48 h. These results suggest that exposure to deltamethrin suppressed the immune responses of M. rosenbergii.