Abstract: In cotton rcot zone,the infection of VAM fungus Glomus mosseae induces a greater change of microbial population and biomass. After infection, the total number of bacteria,actinomycetes,N-fixers and fungi in root zone are decreased at seedling and bud stages,but increased at flowering and boll-forming stages(except that of fungi). In the whole growing period of cotton,the microbial biomass in root zone is remarkably higher after VAM infection.