Abstract: To investigate the specificity of algicidal activity against Scrippsiella trochoidea, Cyclotella striata, Chlorella pyrenoidosa and Scenedesmus quadricauda by bacterial Strain A2, we added Strain A2 to monoculture and mix-culture systems of these four microalage at concentration of 10⁷CFU · mL^-1, and analyzed the algicidal effects. The results show that the Strain A2 was identified as Marinobacter sp. by homology comparisons based on 16S rDNA sequences. Strain A2 produced high lytic effect to S. trochoidea under monoculture and mix-culture systems, and had some algicidal effect to C. striata. The growth of S. quadricauda was not affected by Strain A2. However, Strain A2 could promote the growth of C. pyrenoidosa. Besides, 98.29% and 95.37% of S. trochoidea were killed under the mix-culture systems of four microalage. The cell density of C. pyrenoidosa increased by 237.6%, compared with the control under the mix-culture of S. trochoidea, C. striata and C. pyrenoidosa. It is indicated that Strain A2 has good specificity of algicidal activity to S. trochoidea, and can be potentially used to develop probiotics against dinoflagellate blooms in aquaculture ponds.