Field studies were conducted in 1983 and 1984 to determine the optimal timing of 2,4-D ((2,4-dichlorophenoxy) acetic acid) applications for compatibility with two seed head flies,Urophora affinisFrauenfeld andU. quadrifasciata(Meigen), introduced for control of spotted knapweed,Centaurea maculosaLamarck, in Montana. Application of 2,4-D at the spring rosette stage had no effect on the adult emergence of either fly species in either year. Application of 2,4-D at the flower-bud and flowering stages had no effect on the emergence ofU. quadrifasciatabut significantly reduced the June emergence ofU. affinisduring both years except for the 1984 application at the flowering stage. Integration of the two fly species and the herbicide, 2,4-D, at the rate of 2.25 kg (AI)/ha, for the control of spotted knapweed is possible if the 2,4-D is applied during the spring rosette stage in May.