||Paralytic Shellfish Toxins (PSTs) are a group of potent neurotoxins produced by microalgae including some toxic dinoflagellates and blue-green algae. Recently, increasing findings indicate that some dinoflagellate-associated bacteria are able to produce PSTs independently or to affect the toxin production of dinoflagellates. Bacteria are therefore suggested to be the primary source of PSTs and responsible for toxin synthesis in dinoflagellate cultures. However, to date the issue of bacterial origin of PSTs still remains controversial while the relationship between bacteria and dinoflagellates as regards toxin production remains much to be understood. In this study, the roles of bacteria in the production of PSTs in cultures of two dinoflagellates Alexandrium tamarense, CI01 and HK9301, were investigated. No PST-producing bacteria were found in the two dinoflagellate cultures. In addition, dinoflagellates were demonstrated to produce toxins by themselves under axenic condition. Our resutls thus do not support the hypothesis that bacteria are responsible for the toxins synthesis in the two dinoflagellate cultures. However, the toxicities (toxin production and toxin composition) of the two dinoflagellates were changed when bacteria in dinoflagellate cultures were removed by antibiotic treatments. The results have led us to further investigate the impacts of the remaining bacteria in semi-axenic dinoflagellate cultures on the dinoflagellate toxicity. We found that toxicity of dinoflagellates CI01 and HK9301 could be affected by direct addition of bacteria and the bacterial secretion of water-soluble compounds smaller than 12 kDa into the dinoflagellate cultures. The remaining bacterium HK3 (Ruegeria atlantica) in semi-axenic HK9301 culture could enhance both the growth and toxin production in HK9301 cultures while the remaining bacterium B8 (Pseudomonas stutzeri) in semi-axenic CI01 culture could enhance the toxin production and change the toxin composition in CI01 cultures. The bacterium B8 could also metabolize GTX2. However, whether the metabolizing process is a transformation or degradation remained to be determined.