Bacillus megaterium A6(gusA) colonization in roots of rapeseed in rhizosphere microcosms was investigated by means of gene labeling technique and traditional technique.Rape seeds were inoculated with A6(gusA), an effective strain of bacteria in promoting growth of rape seedlings.Results indicated that A6(gusA) population decreased with the roots going down-wards and with the time after inoculation.Populations of A6(gusA) were not detectable on roots more than 8 cm apart from the hypocotyl base.The population density of A6(gusA) on roots reached the maximum of 87×10⁵cfug^-1 root 3 days after sowing, and then declined rapidly till 30 days when it leveled off at 22×10²cfug^-1 root in rhizosphere microcosms.The result showed A6(gusA) could increased dry weight of the plant and content of total N, total P and total K in the plant.