The impact of benomyl fungicide and spring burning on mycorrhizal activity and plant growth was assessed in tallgrass prairie in Kansas. We report for the first time that the productivity of mycotrophic plants can be reduced by inhibition of indigenous vesicular–arbuscular mycorrhizal fungi under field conditions. A vital stain, nitro blue tetrazolium, used to assess active mycorrhizal colonization, proved to be a more sensitive measure of treatment effects than the cell wall stain, trypan blue. Burning stimulated both plant growth and active mycorrhizal colonization. However, by 32 days after burning no differences in colonization were detected. Our observations support the hypothesis that mycorrhizal fungi play an important role in the growth of warm-season tallgrass prairie grasses and may contribute to enhanced plant growth of warm-season tallgrass prairie grasses and may contribute to enchanced plant growth following spring burning. Key words: burning, benomyl fungicide, phosphorus, tallgrass prairie, VA mycorrhizae, warm-season grasses.