The linden aphid,Eucallipterus tiliaeL. (Homoptera: Callaphididae), is a pest on linden trees,Tiliaspp., which are common as urban shade trees throughout North America. The aphidiidTrioxys curvicaudusMackauer (Hymenoptera: Aphidiidae), which is monophagous onE. tilia, was imported from Europe and released in Berkeley, Calif., where it became established. Subsequent observations showed that the parasite had effectively reduced linden aphid populations below an aesthetic injury level.