The exchange of carbohydrates and mineral nutrients in the arbuscular mycorrhizal symbiosis must be controlled by both partners in order to sustain an evolutionarily stable mutualism. Plants down-regulate their carbon flow to the fungus when nutrient levels are sufficient, while the mechanism controlling fungal nutrient transfer is unknown. Here we show that the fungus accumulates nutrients when connected to a host that is of less benefit to the fungus, indicating a potential of the fungus to control the transfer of nutrients. We used a monoxenic in vitro model of root organ cultures associated with G. intraradices, in which we manipulated the carbon availability to the plant. We found that G. intraradices accumulated up to 7 times more nutrients in its spores, and up to 9 times more in its hyphae, when the carbon pool available to the associated roots was halved. The strongest effect was found for phosphorus, considered to be the most important nutrient in the arbuscular mycorrhizal symbiosis. Other elements such as potassium and chorine were also accumulated, but to a lesser extent, while no accumulation of iron or manganese was found. Our results suggest a functional linkage between carbon and phosphorus exchange.