The overwintering survival of Urophora affinis Frauenfeld and U. quadrifasciata (Meigen) was examined in a 5-yr (1989–1993), multiple-site field study in western Montana. These two tephritid fly species are introduced biological control agents of spotted knapweed, Centaurea maculosa Lamarck, and diffuse knapweed, Centaurea diffusa Lamarck. Significant differences in overwintering mortality of U. affinis and U. quadrifasciata larvae (whether occurring conspecifically or heterospecifically within the same capitulum) were found among sites. Overwintering mortality of U. affinis and U. quadrifasciata (whether occurring conspecifically or heterospecifically within the same capitulum) showed no relationship with site elevation. Overwintering mortality of U. affinis and U. quadrifasciata showed no relationship with the densities of conspecific or heterospecific galls found within the capitula. Extended periods of extremely cold temperature were found to explain most of the variation in overwintering mortality of both fly species. We conclude that periods of extremely cold temperature is the most important mortality factor affecting overwintering survival in U. affinis and U. quadrifasciata.