The distribution of 2 gall flies, U. affinis and U. quadrifasciata, among flower heads of diffuse and spotted knapweed [Centaurea diffusa and C. maculosa, respectively] was determined during a population explosion following their introduction to Canada as biological control agents. The distribution of U. quadrifasciata was more clumped than that of U. affinis and galls of both flies had more clumped distributions in seed heads of spotted knapwee, which has larger heads and more synchronous bud development than diffuse kanpweed. Similar patterns are observed in European populations. On diffuse knapweek U. quadrifasciata galls were more randomly distributed in peak populations and U. affinis galls were more contagiously distributed since the post-introduction peak. Apparently, the equilibrium density of flies is not maintained through adjustment of ovipositional behavior, but is more likely mediated through the response of the plant to fly attack and by environmental factors which influence the phenology of bud development and fly emergence. The introduction of these 2 closely related flies has led to greater seed destruction than would have been achieved by either species alone.