The white shrimp Litopenaeus vannamei were challenged with tryptic soy broth (TSB)-grown Vibrio alginolyticus at a dose of 1 x 10(6) colony-forming units (cfu) shrimp(-1), and then placed in water containing concentrations of nitrite-N at 0 (control), 1.12, 5.15, 11.06 and 21.40 mg l(-1). Mortality of shrimp in 5.15, 11.06 and 21.40 mg l(-1) was significantly higher than those in the control solution after 48-168 h. L. vannamei that had been exposed to control, 0.98, 4.94, 9.87 and 19.99 mg l(-1) nitrite-N for 96 h were examined for THC (total haemocyte count), phenoloxidase activity, and respiratory burst (release of superoxide anion). The THC and phenoloxidase activity decreased when the shrimp were exposed to 4.94, 9.87 and 19.99 mg l(-1) nitrite-N, whereas, the respiratory burst increased significantly at 9.87 and 19.99 mg l(-1) nitrite-N after 96 h. It is therefore suggested that nitrite in water caused a depression in the immune ability of L. vannamei and an increased susceptibility to V. alginolyticus infection, together with an increase of superoxide anion production, possibly to cytotoxic levels for the host.