A suitable host provides, at least, the minimum nutritional and physiological conditions for the development or the immature stages of a parasitoid. Host quality may influence the developmental time, mortality rate, longevity and fecundity of parasitoids. This work evaluates the suitability and quality of Aphis gossypii Glover, Brevicoryne brassicae (Linne), Myzus persicae (Sulzer), Rhopalosiphum maidis (Fitch) and Schizaphis graminum (Rondani) its hosts for Aphidius colemani Viereck. Twenty second-instar nymphs of each aphid species were exposed to parasitism for one hour, and then kept ill a climatic chamber at 22 [plus or minus] 1[degree]C, 70 [plus or minus] 10% RH and a 12 h photophase. The aphid B. brassicae was unsuitable for the development of A. colemani. The different aphid host species varied in size: M. persicae > (R. maidis = S. graminum) > A. gossypii. Parasitoid fitness decreased accordingly when reared on (M. persicae = R. maidis) > S. graminum > A. gossyp! ii. Large hosts seem to be better than small hosts based oil parasitoid size. Egg load of A. colemani was related probably more oil the ability of the parasitoid larva to obtain nutritional resources from the different host species than on host size. Aphidius colemani (Aphidiidae): Parasite; Aphis gossypii (Aphidoidea): Host; Brevicoryne brassicae (Aphidoidea): Host; Myzus persicae (Aphidoidea): Host; Rhopalosiphum maidis (Aphidoidea): Host; Schizaphis graminum (Aphidoidea): Host. Aphidius colemani: Size, parasite biology, host quality and size significance, Fecundity, Development, Development time, Longevity, Emergence, Emergence rate, Hemipteran hosts, Aphis gossypii, Brevicoryne brassicae, Myzus persicae, Rhopalosiphum maidis, Schizaphis graminum, Population sex ratio; Aphis gossypii, Brevicoryne brassicae, Myzus persicae, Rhopalosiphum maidis, Schizaphis graminum: Size, parasite biology, host quality, Hymenopteran parasites, Aphidius colemani. Arthropods; Hymenopterans; Insects; Invertebrates; True Bugs. none