Spurgia esulaeGagné (=Bayeria capitigenaBremi of Solinas&Pecora), a multivoltine gall midge recorded onEuphorbia esulain Italy, was selected as a candidate for the biological control of leafy spurge (Euphorbia esulaL. “complex”) in North America. Its potential host range was studied in 1982 at Rome, Italy, and in 1984 at Albany, Calif., using populations associated withEuphorbia esulafrom San Rossore near Pisa, Italy. Of 56 plant species or varieties in 22 families tested at the Rome laboratory, the midge oviposited and completed its life cycle only on plants in the genusEuphorbia(subgenusEsula). In the tests conducted in Rome, complete development occurred on five populations of North American leafy spurge (from Montana, Nebraska, North Dakota, Oregon, and Saskatchewan) and on EuropeanE. peplusL.,E. cyparissiasL.,E. myrsinitesL.,E. characiasL.,E. serrulataThuillier, and onE. lathyrisL. from Chico and Castro Valley, Calif. Of 12 native North AmericanEuphorbiaspecies tested at Albany in 1984, four species in the subgenusEsula(E. incisaEngelmann,E. palmeriEngelmann,E. robusta(Engelmann) Small, andE. spathulataDe Lamarck) supported the complete development ofS. esulae. The restricted host range suggested use of this midge as a biological control agent against leafy spurge in North America. Releases ofS. esulaewere made in Montana in 1985 and in North Dakota in 1986; this species became established in both states.