The effectiveness of the insecticide triflumuron for controlling the flies Megaselia halterata (Diptera: Phoridae) and Lycoriella auripila (Diptera: Sciaridae), which frequently infect Agaricus mushroom crops, was assessed. Two assays, which involved natural infestation by flies from the cultivation blocks before casing, were carried out. The decrease in the number of first generation adults following application of the insecticide was recorded by sticky traps. The assay was repeated twice with different infestation loads. The possible effect of triflumuron on the Agaricus crop was also evaluated by recording the yield in three successive flushes and the earliness of the harvest. The results show that triflumuron, at both doses used (0.5 and 1 g m-2 active ingredient (a.i.)), does not reduce the yield or delay harvesting. The number of sciarids trapped fell by more than 90% compared with a control, confirming the effectiveness of the insecticide for controlling L. auripila at both doses applied. However, the number of phorids retrieved remained unchanged, regardless of the insecticide dose, so that triflumuron cannot be considered useful for controlling Megaselia halterata.