Two tephritid flies of European origin have been introduced into North America for biological control of spotted knapweed, Centaurea maculosa Lam., and diffuse knapweed, C. diffusa Lam. One, Urophora affinis Frauenfeld, was released in British Columbia in 1970 and in 5 states, including Montana, in 1973 (Hanis 1980a; Maddox 1979; Story and Anderson 1978). The other, Urophora quadrifasciata (Meigen), was introduced at Ned's Creek, British Columbia but was not released in the United States. The larvae of both flies induce galls (1 larva/gall) in immature knapweed flower heads and thus reduce seed production. Galls of Urophora affinis occur in the flower-head receptacle whereas galls of U. quadrifasciata are in the ovary wall. In contrast with U. affinis, which has only a partial second generation, U. quadrifasciata normally has 2 complete generations per year, induces thinner galls, and attacks more-mature flower heads than does U. affinis (Harris 1980a).