The symbiotic bacteria of Alexandrium minutum T1 were isolated and identified as follow: extracelullar species including Pasteurella haemolytica, Sphmon pancimobilis and Pseudomonas vesicularis, and intercelullar species including Pasteurella pneumotropica, Morganella wisconsensis, Flavobacterium oryzihabitans, Pseudomonas pseudomallei and Sphmon pancimobilis. All of them were cultured and determiner to have no PSP-producing ability identified by HPLC analysis. The cell toxicity of A. minutum did not decrease even the culture medium were added with antibiotics. When the dinoflagellate were cultured together with symbiotic bacteria, M. wisconsensis, S. pancimobilis or P. pseudomallei, the growth of A. minutum was better then that with no bacteria at the beginning, but the maximun cell number was lower. The cell toxicity of A. minutum cultured with bacteria was similar to that of A. minutum cultured without bacteria from lag phase to stationary phase, but it is lower after stationary phase.