The predatory activity of mites on immature stages of pest flies was evaluated under laboratory conditions. Two different bioassays were carried out: (1) the effect of prey location (fly eggs) on the predatory ability of Glyptholaspis confusa (Fòa) and Macrocheles muscaedomesticae (Scopoli), and (2) the predation rates of G. confusa and Macrocheles robustulus Berlese on fly immature forms. In the prey location trial, M. muscaedomesticae was an effective predator of both hidden and exposed eggs of Musca domestica L, while G. confusa only showed a significant predatory effect on hidden eggs. This predatory behaviour could be effective for controlling coprophilous pest flies that oviposit underneath the dung pat, such as the horn fly. For predation rate trials, mites were exposed to eggs and larvae of horn fly, Haematobia irritans L., produced naturally. Maximum predation rates on horn fly immatures were obtained with G. confusa deutonymphs, followed by G. confusa females and M. robustulus females. G. confusa deutonymphs can destroy first instar larvae of horn flies, offering another possibility for controlling immature flies in dung. The biggest predators produced the most important impact on fly immature survival. G. confusa was an effective predator of the horn fly under laboratory conditions.