We pre-grew shrimp under polyculture (shrimp-tilapia 'ST' and shrimp-mullet 'SM') and monoculture (P-control) conditions until they reached the size of 10-12 g. Then, the shrimps were collected and injected with Vibrio alginolyticus (2.1 x 10(5) CFU shrimp(-1)). Some shrimp, originating from the shrimp monoculture, was also injected with sterile saline solution, served as N-Control. Over 8 days, mortality in N-control was significantly lower than that in the other treatments (p = 0.001). Total haemocyte counts (THC), phenoloxidase activity (PO) and respiratory burst activity (RB) in haemocytes decreased sharply after injection and were significantly different in P-control, ST and SM as compared to N-control (p < 0.05). However, there were no significant differences in shrimp survival and immune parameters among P-control, ST and SM (p > 0.05). It is concluded that the application of shrimp-fish polyculture systems does not contribute to the robustness of cultured shrimp as verified by challenge through injection. The second experiment was carried out to verify the effect of shrimp-mullet polyculture on the prevalence of Vibrio spp. Mullet (3.45 g) were supplemented (either biomass of 500 or 700 g m(-3)) into six aquaria where shrimp (0.48 g) was stocked at a density of 80 g m(-3). The other six aquaria were just stocked with shrimp as P-control and N-control. Aquaria were inoculated with V. harveyi at a density of 10(3) CFU ml(-1), except for N-control. Results showed that the presence of mullet at either biomass of 500 or 700 g m(-3) could reduce TCBS counts in water by 20.9% and 24.3%, respectively, as compared to shrimp monoculture.