A gall-producing fly,Urophora affinisFrfld., was released at a site in western Montana in 1973 for the control of spotted knapweed (Centaurea maculosaLam.). A total of 279U. affiniswas field-released while 150 were released into a field cage. TheU. affinispopulation within the field cage increased significantly over a 2-yr period as evidenced by an increase of infested seed heads from 15.2–71.3%. Galls of field-releasedU. affiniswere found at a distance of 50 m from the initial release point after 2 yr; adults were found throughout the field in small numbers. The life-history of the fly was closely synchronized with the development of its host plant.