Impact of the presence of bacteria associated with a marine dinoflagellate, Alexandrium tamarense CI01, on the growth and toxin production of the algae in batch culture was investigated.Pronounced changes in the activities of the algal culture were observed when the culture was treated with different doses of a mixture of penicillin and streptomycin.In the presence of antibiotics at the initial concentration of 100 u/mL in culture medium, both algal growth and toxin yield increased markedly. When the concentration of antibiotics was increased to 500 u/mL, the microalgal growth was inhibited, but resumed in a few days to eventually reach the same level of growth and toxin production as at the lower dose of the antibiotics. When the antibiotics were present at a concentration of 1 000 u/mL, the algal growth was inhibited permanently.The results indicate that antibiotics can enhance algal growth and toxin production not only through their inhibition of the growth and hence competition for nutrients, but also through their effects on the physiology of the algae.