Trioxys curcicaudus Mackauer was imported into northern California in 1970 for control of the linden aphid, Eucallipterus tiliae (L.). In 1991, it was found no farther than 40 km from the release sites, but in this area it is the dominant primary parasitoid attacking the aphid. This limited distribution is attributed mainly to the scarcity of favored hosts ; climatic conditions play a contributory role. In 1992, T curvicaudus was introduced and established in San Rafael, partially displacing the previously dominant T pallidus (Haliday) ; however, it did not provide satisfactory control of the aphid. Five other primary parasitoids were reared from the linden aphid : T tenuicaudus Stary, Aphelinus subflavescens (Westwood), A. sp. nr. perpallidus Gahan, A. automatus Girault, and an undescribed Trioxys species (the latter three are new host records). Ten hyperparasitoid species were recovered, with new host records for Pachyneuron calfornicum Girault and Coruna clavata Walker on Trioxys, Alloxysta xanthopsis (Ashmead) on Aphelinus, and a species from the Alloxysta megourae (Ashmead) complex on Trioxys and the subgenus Aphelinus (Mesidia). Hyperparasitoid species ranged from patchy to well distributed. Syrphophagus aphidivorus (Mayr) was reared at a significantly higher rate from Aphelinus hosts than Trioxys hosts.