The influence of cultivars of common cabbage, Brassica oleracea var. capitata with varying levels of resistance to Brevicoryne brassicae (L.) and Myzus persicae (Sulzer) on key biological characteristics of Aphidius colemani (Viereck) was investigated under laboratory conditions. The total development time for female parasitoids reared on M. persicae did not differ significantly between Minicole (green-leaved, partially resistant with antibiosis factors for B. brassicae) and Derby Day (green-leaved, susceptible to both aphid species); but development was significantly faster (ca 10%) on Ruby Ball (red-leaved, partially resistant with antixenosis factors for B. brassicae). Total development time for females reared on B. brassicae was slightly shorter on Ruby Ball than on Minicole. Males reared on M. persicae developed into adults significantly faster (ca 10%) on Ruby Ball than on Minicole. However, when B. brassicae was the host, no significant variations in development time were observed. Sex ratios, size and longevity of both male and female parasitoids on either host were not significantly influenced by cultivar. The results are discussed in relation to the compatible utilisation of host-plant resistance and biological control in the integrated management of aphids. Aphidius colemani (Aphidiidae) : Parasite Brevicoryne brassicae (Aphidoidea) : Host, Parasite Myzus persicae (Aphidoidea) : Host, Parasite Animals and man; Control; Biological control; Biometrics; Life cycle and development; Parasites diseases and disorders; Parasites; Insect parasites; Hosts; Insect hosts; Ecology; Population dynamics; Population structure Aphidius colemani; Control of; Brevicoryne & Myzus (Hemiptera); Size; Life cycle and development; Hemipteran hosts; Parasite development; size & sex ratio; effects of plant cultivars with varying levels of resistance; Brevicoryne brassicae & Myzus persicae; Population sex ratio Brevicoryne brassicae; Myzus persicae; Control by; Aphidius colemani (Hymenoptera); Hymenopteran parasites; Parasite biology; significance of varying levels of plant cultivar resistance; Aphidius colemani; Plant hosts; Brassica oleracea var. capitata none