This paper first report the distribution and role of four nitrogen cycle bacteria in the artificial ecosystem of Taihu Lake. These nitrogen cycle bacteria include ammonifying bacteria, denitrify-ing bacteria, nitrobacteria and nitrovsobacteria, determined with MPN method. 1 he MPN values of denitrifying bacteria in the water covered by Alternanthera philoxeroides and Eichhornia cras-sipes are about 2-3 and 2-5 oraer-of-magnitudes higher than the water vegetated with other macrophytes and unvegetated water respectively. There is significant difference between them (P <0.01). The MPN values of ammonifying bacteria in vegetated water are markedly higher than that in unvegetated water. The MPN values of nitrosobacteria in vegetated water are notably higher than those in unvegetated water (P<0.01). The MPN values of nitrobacteria and ni-trosobacteria in the water vegetated with submerged plant are nigher than those in the water cov-ered by floating plant. The MPN values of ammonifying and denitrifying bacateria, nitrosobacte-ria are very dense in rhizosphere, and decrease from rhizosphere to outside.