Previous studies have indicated that although the cabbage aphid, Brevicoryne brassicae (Linnaeus) Homoptera/Aphididae, and the green peach aphid, Myzus persicae (Sulzer) Homoptera/Aphididae), are both suitable hosts for the aphid parasitoid, Diaeretiella rapae (McIntosh) Hymenoptera/Aphidiidae, D. rapae prefers B. brassicae because it oviposits more frequently and produces a significantly higher percentage of progeny in B. brassicae. We developed life table parameters for D. rapae reared on B. brassicae and M. persicae to determine whether there really were differences in the suitability of these two hosts. An examination of key life history parameters indicated that M. persicae is the better host of the two. Net replacement rate, birth rate, intrinsic, and finite rates of increase were all higher in D. rapae that developed in M. persicae than in B. brassicae. Although generation time was slightly shorter for D. rapae reared on B. brassicae than on M. persicae, doubling time was also shorter for D. rapae reared on M. persicae compared to B. brassicae. A population model based on demographic data indicated that D. rapae populations would grow much faster when developing on M. persicae compared to B. brassicae. Because D. rapae reared on M. persicae had higher intrinsic rates of increase and shorter doubling time than when reared on B. brassicae, M. persicae is a better host for this parasitoid at least under the environmental conditions of this study. These data also indicate that D. rapae should be a more effective biological control for M. persicae than for B. brassicae.