Competitive interactions between the two natural enemies of Icerya purchasi—Rodolia cardinalis and Cryptochaetum iceryae—were studied in detail in order to throw more light on the question of the introduction of multiple natural enemies as a biological control policy. The host, a major citrus pest accidentally introduced in 1868 from Australia, has been under control since 1888-1889, when its highly specific natural enemies (also from Australia) were imported for that purpose. Three different climatic regions of southern California were the study sites: the desert, the coast, and the interior. Life table studies of the scale indicated that it is maintained under control at very low population levels in the desert by vedalia and on the coast by Cryptochaetum, with competition causing displacement in both areas. In the interior, the two enemies seasonally share their prey in different proportions and are fairly even in their competitive abilities—vedalia usually taking more prey during the summer and fall, the fly taking more during the winter and early spring. In the interior, three generations of the host occurred in a year, while those of vedalia and Cryptochaetum averaged 12 and 8, respectively. Some host plants restricted vedalia development, and certain species of ants interfered with Cryptochaetum. Competition results in micro-habitats also varied, depending on prevailing environmental conditions and the time at which an enemy finds a scale colony. The “coexistence” of vedalia and Cryptochaetum in the interior is thought to be only apparent and maintained by the dispersal of Cryptochaetum from their coastal area of dominance. Coexistence experiments in the interior showed displacement of the fly by vedalia to occur after nine generations. Competition between the two natural enemies did not increase host survival. These extensive studies provided strong support for the policy of importation of multiple natural enemies.