Abstract: Many kinds of amino acids are found in root exudates from Eichhornia crassipes. F₂ can be strongly attracted to amino acids methionine, gamma amino butyric acid, glycine, alanine, aspartate, serine, valine, and leucine under concentrations from 10^-7 to 10^-2 mol·L^-1, some degrees to glutamate, threonine and histidine, but not to lysine, cysteine,arginine, tyrosine, proline, asparagine, glutamine, isoleucine, phenylalanine or tryptophan. There exists an optimium chemotaxic concentration range of each positive attractant. The fact that some amino acids caf f attract the bacteria,but others can not is one of the reasons why Eichhornia crassipes and F₂, can form a microecosystem.

Key words: Eichhornia crassipes, Enterobacter sp. F₂, Amino acids, Chemotaxis, ecological water demand, hydrological variation, the southern three outlets of Jingjiang River