The native parasitoid complexes associated with four key muscids, Musca domestica L., Stomoxys calcitrans (L.), Fannia canicularis (L.), and F. femoralis Stein—and two predatory Diptera, Muscina stabulans (Fallen) and Ophyra leucostoma (Wiedemann)—are described and shown graphically for the major climatic zones of the southwestern United States. Three geographical groups of the parasitoid species identified were (1) those whose populations extended into most climatic zones below 2,000 meters (Muscidifurax species and Spalangia nigroaenea Curtis); (2) those whose populations were restricted to the warmer climatic areas (S. endius Walker and S. cameroni Perkins); and (3) those that were largely restricted to higher latitudes and colder winter climates (S. nigra Latreille and Aleochara spp.). Considerable yearly, seasonal, and climatic variations were shown in the abundance of both host and parasitoid demes. These factors, combined with others, probably account for the high degree of instability among demes. This instability may be important in determining the panmictic nature of most populations. Some parasitoids such as the Diapriidae and Staphylinidae (Aleochara spp.), which exist in small demes, appeared to be local species that may have adapted to invading Diptera. The Stilpnus species were specific to Fannia. Parasitoid species appear to be irreplaceable mortality factors in the natural control of populations of their dipterous hosts.