The great variability of the aphid life cycle and their tendency for host alternation gives rise to aphid clones and morphs. Inter- and intraclonal variability may be observed in the responses of aphids to various environmental factors. In this study we aimed to evaluate the influence of intrinsic factors (clone and morph type) on the intrinsic rate of increase (rm) of the English grain aphid, Sitobion avenae (Fabricius), and the bird cherry-oat aphid, Rhopalosiphum padi (Linnaeus). For each species four apterous clones were collected from established laboratory colonies and compared to assess their relative fitness on high- and low-nitrogen wheat plants under laboratory conditions. The clones had significantly different intrinsic rates of increase on high- and low-nitrogen plants. All R. padi clones had a higher intrinsic rate of increase and mean relative growth rate than S. avenae. Experiments were also conducted to compare the mean fecundity of apterous and alate morphs of S. avenae and R. padi clones on high- and low-nitrogen wheat plants. On high-nitrogen plants the apterous morphs of S. avenae clones had significantly higher mean fecundity than alate morphs. There were no significant differences between the mean fecundity of alate morphs of the same species on high- and low-nitrogen plants. The results support the idea of better fitness of specific clones/morphs on certain host plants due to higher and lower intrinsic rates of increase.