Field placement of cocoons of Operophtera spp. was used to determine the timing of attack by pupal parasitoids of the winter moth, Operophtera brumata (L.), and the Bruce spanworm, O. bruceata (Hulst). Coccygomimus hesperus Tow. & Tow., the most abundant parasitoid recovered, attacked Operophtera pupae from early June until the end of August. At least two generations of C. hesperus occur each season. Buathra dorsicarinata (Pratt) was not recovered in numbers large enough to determine its timing of attack and no pupae parasitized by Cratichneumon sp. were recovered