Chromosomal races of the aphid Sitobion miscanthi (Takahashi), and a closely-related species S. near fragariae (Aphididae: Macrosiphini) show seasonal differences in abundance in eastern Australia. We tested whether this was related to differences in temperature response, using controlled laboratory conditions. Intrinsic rate of increase, rm, mean relative growth rate (MRGR), and longevity were compared at different temperatures, and rates of increase were compared outdoors under ambient conditions. Using laboratory data, we simulated population growth, for comparison with outdoor populations. There were significant differences in responses to temperature regimes both in the laboratory and outdoors. Outdoor populations fell well short of predicted sizes. S. miscanthi (2n=18) showed evidence of adaptation to warmer conditions. In contrast, S. nr fragariae showed consistent evidence for adaptation to cooler conditions, and S. miscanthi2n=17 was approximately intermediate. These data indicate that temperature responses are important in the ecology, evolution and pest status of S. miscanthi clones and S. nr fragariae in Australia.