The gall midge Obolodiplosis robiniae (Haldeman) (Diptera Cecidomyiidae), originating from North America and damaging to black locust (Robinia pseudoacacia L.), was detected for the first time in Europe (northern Italy) in 2003. In the subsequent years, the distribution of this O. robiniae in Europe has rapidly expanded and severe symptoms on plants were observed in a number of geographic areas. In Italy, O. robiniae populations are affected by various natural enemies, in particular by a parasitoid in the family Platygastridae. The species involved in parasitism, Platygaster robiniae, is described in this article.