Because the lettuce root aphid, Pemphigus bursarius (L.), is gall-inhabiting on poplar in spring and lives underground on lettuce during summer, its naturalenemy complex differs from that associated with foliage-feeding aphids. It suffers little from parasites and only fundatrices have been found attacked. The only species reared from parasitised individuals was the Pteromalid, Pachyneuron sp., which is assumed to be hyperparasitic. When the galls open at migration time, Syrphid larvae and Anthocorids can enter and attack the contents. Anthocoris nemorum (L.) and A. nemoralis (F.) are the main predators at this stage. Before they begin feeding, they kill all the aphids within a gall extremely quickly and efficiently. It is suggested that the aphids may be killed by a secretion from the stink glands of the Anthocorids working as a fumigant within the confines of the gall. Several Staphylinids and Carabids are frequently associated with subterranean colonies of P. bursarius and are almost certainly predators. The larvae of the Chloropids, Thaumatomyia glabra (Mg.) and T. notata (Mg.), are common predators of P. bursarius in the soil. The sexuparae, both before they leave the lettuce plants and on their arrival on poplar, are attacked by three species of Coccinellid.