Phosphatidylcholine, phosphatidylglycerol, phosphatidylethanolamine and cardiolipin are the major phospholipids in young leaves of black oak (Quercus robor L.). Except for phosphatidylcholine, young, developing cynipid-galls on black oak leaves, i.e. the insect-transformed tissues, contain less phospholipid than normal leaf tissues. Lipid acyl hydrolase activity determined by the cleavage of free fatty acids from a labeled phospholipid substrate is higher in the tissue extracts from galls than from leaves. The increase in enzyme activity and the altered phospholipid composition are discussed in relation to expected membrane modifications and transport phenomena in insect-transformed tissues.