To study the effect of alkaline phosphatase(ALP) bacteria on Microcysfis aeruginosa obtaining phosphorus from organic phosphorus compounds, an attached bacterium strain Y6 which produced ALP was isolated from M.aeruginosa colonies.It was identified as a strain of Erythrobacter by its ITS molecular characteristic.The effects of the bacterium on the growth and physiology of M.aeruginosa in the organic phosphorus condition investigated in this paper.The results show that:(1) ALP activity in the culture broth was positively related Y6 concentration, initial lag phase of incubation began to produce ALP;(2) Under the condition of organic phosphorus, Y6 could promote the growth of M.aeruginosa;(3) Y6 could maintain floating rate and bigger colonies of colonial M.aeruginosa, and hold chlorophyll fluorescence of algal cells in the organic phosphorus medium.This research suggested that ALP producing attached bacteria could help M.aeruginosa using organic phosphorus and forming blooms.