A Cecidomyiid gall midge, Cystiphora schmidti (Rübsaamen), lives on several related Chondrilla spp. occurring from Greece and eastern Europe to Iran. Its biology and host specificity were studied in the eastern Mediterranean region as part of the biological control programme against the weed C. juncea (Compositae, Cichoriaceae) in Australia. The adult female oviposits into the lower epidermis of the plant where small circular to ovoid raised galls are produced on the rosette, stem leaves and the stem. All the immature stages are completed inside the gall in 24–44 days, allowing 6–7 generations per year in southern Greece before low temperatures prevent development of overwintering larvae. The midge is heavily parasitised in Greece by two species of Eulophidae. Particularly in southern Greece, dense gall populations cover the leaves and stems of C. juncea, causing death of leaves, stunting and less seeding. Tests against 76 species of plants including some Cichoriaceae closely related to Chondrilla showed that Cystiphora schmidti is specific to the genus Chondrilla. Comparative tests with Cystiphora schmidti from Greece against four forms of Chondrilla juncea showed that the midge was adapted to its usual Greek host but was less closely adapted to a western Mediterranean form. The Greek strain of the midge readily attacked and thrived on the main Australian form of C. juncea.