Significant reductions of the average densities of 7 Diptera : Musca domestica L.,[emoralis (STEIN}, Ophyra leucostoma (WIEDEMANr~), Stomoxys calcitrans IL.}, and Phaenicia spp. were attained over a 20-month period on poultry ranches that were under supervision in southern California by favoring the natural increase of predatory and scavenger arthropods and by periodic inoculative releases of 4 parasitic Hymenoptera. The importance of a long range management plan is emphasized. Releases of parasitic Hymenoptera during spring months had an apparent greater direct effect on fly population reduction than did similar releases in the summer. Autumn releases were not evaluated through a temporary ban on sampling breeding habitats in the wake of a Newcastle Disease outbreak. The importance of habitat stability and exotic importations of beneficial species are discussed.