Ant predation by two species of Iridomyrmex had a major impact on juvenile survival of Pieris rapae populations in the Canberra area. Exclusion experiments demonstrated that ant predation fell most heavily on the later instars, and that in the absence of ant predation, survival of instars III-V was very high. P. rapae larvae parasitized by the braconid parasite Apanteles glomeratus were more vulnerable to ant predation than were unparasitized larvae. Thus, the effects on ant predation and of parasitism are not cumulative. The generality of discrimination by predators between parasitized and unparasitized insect prey is discussed.