The effects of rhamnolipid biosurfactant produced by Pseudomonas aeruginosa on the growth and the mitigation inhibitory of Alexandrium tamarense,Heterosigma akashiwo and 3 kinds of diatom ( Chaetoceros Didymus,Chaertoceros debilis and Nitzschia closterium) were investigated in the laboratoray.As the concentration of rhamnolipid was increased gradually from 0.5 to 12.0 mg/L,it displayed different degree of inhibitory action on the growth of 5 kinds of algae.The reason for the different susceptibilities of the algae to the rhamnolipid concentration was investigated according to the analysis of the difference in the algae fatty acid composition.This different character was interpreted by the different fatty acid composition of the algae biomembrane,the lower the content of polyunsaturated fatty acids was,and the lower the corresponding 96h - EC50 value was,the same on the contrary.The optimal concentrations of rhamnolipid required for the effective mitigation of the five algal species ranged from 8.5to 15 nig/L.The nucleotides of algal species were evaluated when the rhamnolipid was added in research as the release of intracellular components is a good indicator of membrane integrity.The results indicated that the algal cells were irreversibly damaged by the rhamnolipid.