Field and laboratory studies were conducted to determine how Apanteles scutellaris and Agathis gibbosa (Hymenoptera: Braconidae) coexist as solitary endophagous parasitoids of potato tuberworm (PTW) larvae, Phthorimaea opercullela (Lepidoptera: Gelechiidae), in southern California. The competitive characteristics of Orgilus jennieae (Hymenoptera: Braconidae), an exotic parasitoid of PTW larvae, were compared with those of the native species to determine its potential for establishment. Emergence of the egg-larval parasitoid Chelonus phthorimaeae (Hymenoptera: Braconidae) from field-collected PTW larvae was compared with those of the other parasitoids, but its competitive interactions were not considered. Seasonal density changes, mine characteristics, and within-plant distributions of PTW larvae were studied to determine their possible impacts on parasitoid interactions.