The PSP toxicity of Alexandrium tamarense (Lebour) Balech on the blue mussel (Mytilus edulis) from the western waters of Xiamen in China was studied by following the standard method of PSP mouse bioassay developed by the Association of Official Analytical Chemists (AOAC). The results showed that the mice survived when the density of A. tamarense cells was lower than 1×105 cells per cubic decimetre and died when the density was higher than 1×106 cells per cubic decimetre. The past record of red tide events in the western waters of Xiamen showed a general trend of starting from the bloom of non-toxic planktonic diatoms in local waters and resulting in a harmful algal bloom due to the fade of planktonic diatoms which failed in the survival competition in the unfavorable and deteriorated eco-environment. On the basis of experimental results and natural environment of Xiamen waters and by making reference to the critical criteria of shellfish toxins in various states, a prewarning value 1×105 cells per cubic decimetre of A. tamarense PSP toxicity was proposed for the areas in South China where red tides frequently occur. The PSP toxicity of Alexandrium tamarense (Lebour) Balech on the blue mussel (Mytilus edulis) from the western waters of Xiamen in China was studied by following the standard method of PSP mouse bioassay developed by the Association of Official Analytical Chemists (AOAC). The results showed that the mice survived when the density of A. tamarense cells was lower than 1 × 105 cells per cubic decimetre and died when the density was higher than 1 × 106 cells per cubic decimetre. The past record of red tide events in the western waters of Xiamen showed a general trend of starting from the bloom of non-toxic planktonic diatoms in local waters and resulting in a harmful algal bloom due to the fade of planktonic diatoms which failed in the survival competition in the unfavorable and deteriorated eco-environment. On the basis of experimental results and natural environment of Xiamen waters and by making reference to the critical criteria of shellfish toxins in various states, a prewarning value 1 × 105 cells per cubic decimetre of A. tamarense PSP toxicity was proposed for the areas in South China where red tides frequently occur.