Four species of agromyzid leafminers, Agromyza frontella (Rondani), A. nana Meigen, Liriomyza congesta (Becker), and Phytomyza horticola (Goureau), the last representing a new host plant record, were studied in selected alfalfa fields in France and Denmark in 1977. Only 0.4% of the alfalfa leaflets contained agromyzid mines, a degree of infestation that is far below the 25 to 30% level or higher that may occur in alfalfa fields in the northeastern United States and Canada. The combined action of hymenopterous parasites of the families Braconidae, Eulophidae, and Pteromalidae produced an overall seasonal parasitism (determined by dissection) in excess of 70% for all hosts species. In some samples, parasitism reached 100%. New host records were established for 16 of the 17 species recovered from A. frontella and 11 of the 18 species from A. nana. The sequence of parasite species, multiple and superparasitism, competition, encapsulation by the host, and general geographical distribution in Europe were also studied. A total of 14 species were sent to the United States for study and release against A. frontella, a pest recently introduced into North America. Of these, three are now established in the United States.