The benefits of mutualistic interactions are often highly context dependent. We studied the interaction between the milkweed aphid Aphis asclepiadis and a tending ant, Formica podzolica. Although this interaction is generally considered beneficial, variation in plant genotype may alter it from mutualistic to antagonistic. Here we link the shift in strength and relative benefit of the ant?aphid interaction to plant genotypic variation in the production of cardenolides, a class of toxic defensive chemicals. In a field experiment with highly variable genotypes of the common milkweed (Asclepias syriaca), we show that plant cardenolides, especially polar forms, are ingested by aphids and excreted in honeydew proportionally to plant concentrations without directly affecting aphid performance. Ants consume honeydew, and aphids that excreted high amounts of cardenolides received fewer ant visits, which in turn reduced aphid survival. On at least some plant genotypes, aphid numbers per plant were reduced in the presence of ants to levels lower than in corresponding ant?exclusion treatments, suggesting antagonistic ant behavior. Although cardenolides appear ineffective as direct plant defenses against aphids, the multi?trophic context reveals an ant?mediated negative indirect effect on aphid performance and population dynamics.