(1) The benefit and loss provided to oak trees by wood ants were assessed in a Quercus dentata chaparral located in a supercolony of the red wood ant, Formica yessensis. (2) Ants decreased the numbers of leaf feeders such as lepidopteran larvae and weevils, thereby reducing leaf loss; however, aphid numbers increased in the presence of ants. (3) Acorn production was similar on trees with and without ants, but the percentage of acorns not infested by acorn borers was much higher on ant-abundant trees (48.8%) than on trees where ants were infrequent (27.7%). (4) These results support the presence of an indirect mutualism between oaks and wood ants via myrmecophilous aphids.