Trioxys curvicaudusMackauer 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. curvicauduswas introduced and established in San Rafael, partially displacing the previously dominantT. pallidus(Haliday); however, it did not provide satisfactory control of the aphid. Five other primary parasitoids were reared from the linden aphid:T. tenuicaudusStary,Aphelinus subflavescens(Westwood),A sp. nr.perpallidusGahan,A. automatusGirault, and an undescribedTrioxysspecies (the latter three are new host records). Ten hyperparasitoid species were recovered, with new host records forPachyneuron califonicumGirault andCoruna clavataWalkeronTrioxys, Alloxysta xanthopsis(Ashmead) onAphelinus,and a species from theAlloxysta megourae(Ashmead) complex onTrioxysand the subgenusAphelinus(Mesidia). Hyperparasitoid species ranged from patchy to well distributed.Syrphophagus aphidivonls(Mayr)was reared at a significantly higher rate from Aphelinus hosts thanTrioxyshosts.