Vesicular-arbuscular mycorrhizal (VAM) fungi colonize plant roots and the surrounding bulk soil. They transport mineral nutrients from the soil to the plant and carbon compounds from the plant to the soil, and have pervasive effects on plant form and function and on the composition of the soil microbiota. This experiment evaluated VAM effects on plants and soil to determine if VAM fungi mediate a relationship between changes in seed yield and soil aggregation. In a pot experiment with peas, an isolate of the VAM fungus Glomus mosseae (Nicol & Gerd.) Gerd. and Trappe did not significantly affect seed yield (8%), but improved soil aggregation by 400% in one soil, a gray silt-loam high in organic matter (OM) and phosphorus. In another soil, a yellow clay-loam low in OM and phosphorus, seed yield was enhanced significantly (57%), but there was only a small change (50%) in aggregation. The results suggest that carbon allocation between the plant (measured as seed yield) and the soil (measured as the formation of water-stable aggregates) is influenced by this VAM fungus. The soil appeared to gain carbon at the expense of carbon lost by the plant Mycorrhizal fungi thus seem to affect two biologically controlled aspects of sustainable agriculture: plant production and soil quality.