Abstract: The inhibitory effect of different density of Chaetoceros muelleri on the vibrios was studied in this paper. The results show that microalgae culture system can inhibit the growth of vibrios. And the more algae density is, the better the inhibitory effect is. The inhibitory effect of microalgae culture system to Vibrio alginolyticus is stronger than that to V. parahaemolyticus. Vibrios can make use of algae nutrition and grow rapidly, but can't promote or inhibit algae cell from growing. In the microalgae culture system, the growth of heterotrophic bacteria varied with the density of algae. But at the latter stage of microalgae's growing, the quantity of heterotrophic bacteria reached a stable value (about 2105 CFUmL-1) and the number of strain A was far beyond the total of other heterotrophic bacteria. A mutual growth existed in strain A and chaetoceros because of the high affinity.