Field studies were conducted in western Montana during 1985 and 1986 to determine the voltinism, sex ratio, and the phenological synchrony ofUrophora affinisFrauenfeld andU. quadrifasciata(Meigen), two flies introduced for biological control of spotted knapweed. Varying percentages of adults of both fly species emerged in late July through September for a second generation. Mean percentage second-generation emergence ofU. affiniswas 7.1 compared with 35.2 forU. quadrifasciata. Percentage of second-generation emergence for both fly species was highest in the earliest maturing seed heads and decreased gradually in later-maturing seed heads. Ninety percent of second-generation emergence occurred within 2–3 wk of seed-head maturation. A small number of both fly species emerged in September for what may be a third generation. First-generation emergence ofU. affinisandU. quadrifasciatapeaked on 25 June and 2 July, respectively, while peak second-generation emergence of both fly species occurred on 19 August. Peak first-generation emergence of females of both fly species from previous year seed heads of spotted knapweed occurred when the majority of current-year flower heads were 2–4 mm in diameter, while second-generation emergence occurred immediately after the infested seed heads had opened for seed dispersal. Males of both fly species generally emerged earlier than females in both emergence periods, but the male-to-female ratio in the first half of the emergence periods was higher forU. affinisthan forU. quadrifasciata. The distinct differences in voltinism and associated survival strategies serve as key factors in the niche separation of the two species. AlthoughU. affiniswill likely become the dominant species at sites where the two flies coexist, the two flies' niche separation will probably ensure the continued presence ofU. quadrifasciatain North America.