This study investigated whether Lathrolestes ensator, a parasitoid released as a biological control agent against the European apple sawfly in Canada, is successfully attacked by the cocoon parasitoid A.nigrocincta. In a no-choice situation in the laboratory, there was no significant difference between the acceptance of unparasitized hosts and hosts containing an egg of L. ensator by females of A.nigrocincta. In addition, A.nigrocincta attacked mature larvae of the larval parasitoid to the same degree as they attacked unparasitized hosts. Survival of A.nigrocincta offspring was close to 100% regardless of whether they developed in unparasitized hosts, in hosts containing an egg of L. ensator or in host cocoons containing mature larvae of L.ensator. This is discussed in the context of the possibility of releasing A.nigrocincta as an additional biological control agent of the European apple sawfly.