Bacteria can freely grow in the phycosphere of marine alga which produce extracellular substances to their surrounding environment. These bacteria could regulate the growth of algae. One percent (v/v) of 2216E medium was mixed into Alexandrium tamarense culture system. These results show that algal cells were all lysed within 16 hours. After adding 2216E medium, both the abundance and the extracellular enzyme activity of bacteria increased by 50-100 times from the 6th hour to the 10th hour in lysis of algae. These experiments suggest that bacteria in the phycosphere could mainly attach to the surface of Alexandrium tamarense. Changes in the population of this community might cause the lysis in algae. The β-D-glucosidase and chitinase produced by bacteria could play a major role in lysis of algae.