AbstractThe mutualism between plants and arbuscular mycorrhizal fungi (AMF) is widespread and has persisted for over 400 million years. Although this mutualism depends on fair resource exchange between plants and fungi, inequality exists among partners despite mechanisms that regulate trade. Here, we use 33P and 14C isotopes and a split‐root system to test for preferential allocation and reciprocal rewards in the plant–AMF symbiosis by presenting a plant with two AMF that differ in cooperativeness. We found that plants received more 33P from less cooperative AMF in the presence of another AMF species. This increase in 33P resulted in a reduced 14C cost per unit of 33P from less cooperative AMF when alternative options were available. Our results indicate that AMF diversity promotes cooperation between plants and AMF, which may be an important mechanism maintaining the evolutionary persistence of and diversity within the plant–AMF mutualism.