The relationship between Alexandrium tamarense(Lebour) Balech, one of red-tide alga, and two strains of marine bacteria,Bacillius megaterium(S7 ) and B. halmapulus(S10) isolated from Xiamen Western Sea, was investigated by evaluating the growth state of A. tamarense and the variation of β-glucosidase activity in co-culture system. The results showed the growth and multiplication of the alga were related with the concentration, genus speciality of the bacteria, and growth stage of the alga itself. The growth of A. tamarense was obviously inhibited by S7 and S10 at high concentration. Either inhibition or promotion contributed much more clearly in earlier than in later stage of the growth of the alga. Furthermore, there was a roughly similar variation trend of the activity of extra-cellular enzyme, β-glucosidase, in the water of the separately co-cultured bacteria S7 and S10 with the alga. The β-glucosidase activity(β-GlcA) rapidly increased during the later algal growth accompanying the increase of the lysis of the alga cells. The obvious inhibition of A. tamarense by marine bacteria at high concentration and evident increase of β-GlcA in co-colture system would help us in better understanding the relationship between red-tide alga and bacteria, and also enlightened us the possible use of bacteria in the bio-control of red-tide.