Galls produced by Andricus symbioticus supply ants with a sweet and sticky food. Interactions between the gall-attending ant, Lasius niger and the parasitoid, Caenacis sp. (Family Pteromalidae) were observed on the galls of A. symbioticus on Quercus mongolica var. grosseserrata at Chojabaru, Kuju Mts. of Oita Prefecture, Japan. On the basis of this observation the association between the gall and the gall-attending ant is regarded as trophobiosis.