Abstract: To evaluate the harmfulness of ammonia on Pacific white shrimp (Litopenaeus vannamei), we investigated the effects of ammonia (NH₄-N) on mortality of the shrimps infected with WSSV (white spot syndrome virus), WSSV proliferation and activity of immune-related enzymes in serum. The shrimps were infected intramuscularly with 2×10⁵ and 2×10⁶ viral particles under the stress of 15.6 mg · L^-1 NH₄-N. The results show that the shrimps with 2×10⁵ WSSV got mortality of 53.3% at 144^th hour, which was significantly higher than that of the control group (stress-free). Meanwhile, the real-time relative PCR shows that ammonia stress accelerated WSSV proliferation in shrimp gills. In addition, it is showed that, under ammonia stress, the activities of immunity-related enzymes such as phenoloxidase (PO), acid phosphatase (ACP) and alkaline phosphatase (AKP) in shrimp serum continued to decrease after a brief rise. The results indicate that ammonia stress accelerates WSSV proliferation in shrimps and causes higher mortality, which is related to the decrease of activities of immune-related enzymes under ammonia stress.