ABSTRACTThe ability of eight vesicular-arbuscular mycorrhizal (VAM) fungi to stimulate growth of the warm-season prairie grassAndropogon gerardiiwas compared in steamed prairie soil, steamed soil amended with non-sterile soil sievate, and in non-sterile soil. Five of the VAM fungi were highly effective in steamed soil, i.e., inoculated plants were 56–224 times larger than noninoculated plants. However, mycorrhizal growth response from these five species was suppressed by an average of 23.6% in the presence of soil microbes, i.e., either in steamed soil with sievate or in non-sterile soil. The three VAM fungi which produced little or no growth response were not subject to this microbial suppression. Percentage of root colonization produced by the eight VAM fungi was highly variable, ranging from 17.3 to 78.2%, and was not correlated with growth response. In general, species which produced extremely high colonization rates in steamed soil were inhibited from doing so in the presence of the soil microflora. Apparently, regardless of VAM fungus species, the soil microflora regulates degree of growth response and root colonization. Metalaxyl fungicide was added toA. gerardiiandSchizachyrium scoparium(=Andropogon scoparius) plants in the three soil treatments. While the fungicide increased root colonization in non-sterile soil, there was no corresponding increase in plant dry weight. The fungicide may allow mycorrhizal fungi to proliferate in roots, but control of oomycete fungi does not fully eliminate the suppressive effect of the soil microflora on mycorrhizal growth response.