The final instar larva of Synergus filicornis (Cameron) (Hymenoptera: Cynipidae: Synergini) is described and illustrated. Morphological structures of a diagnostic value are discussed. The most remarkable character states shown by the mature larva of this species lie in the integument, which unlike the rest of described cynipoid mature larvae, presents tegumental differentiations. Additionally the antennal orbits, unlike those of other described Synergini mature larvae, did not present one seta on each antennal orbit. The immature larvae can be differentiated from the mature larva by the following characters: a) number and location of sensorial tegumental differentiations on the head, b) number of sensilla on the maxillary palpi, and c) morphology of the mandibles. The determination of the mandibles size, in this species, permits the estimation of the larval stages number.