In this study, we investigated the effects of temperature on body color and on the population dynamics of grain aphids Sitobion avenae (F.). Populations of aphids were reared using two methods: a) on wheat grown in pots, and b) single head on wheat in Petri dishes. Five constant-temperature regimes were established: 21, 23, 26, 29 ℃ and 31 ℃. In each of the treatments, measurements were made of the total numbers, and the relative proportions, of red and green aphids in the population. We also recorded the fastest time for reproduction of the population, the average generation time, and the stable age-distribution of the red and green aphid populations. Red aphids were present in the populations at 23 ℃ and at higher temperatures; the relative proportion of red aphids in the population progressively increased with each increase in temperature. However, there were clear differences in the effects of temperature on the numbers of red and green aphids in the populations. High temperatures enhanced the production of the red aphid biotype but suppressed the numbers of green aphids so that the overall population size decreased. Although, at 31 ℃, the proportion of red aphids in the whole population was very high, the total number of red aphids in the population was lower at this temperature than at 26 ℃ or at 29 ℃. This indicates that very high temperatures suppressed the population growth of both red and green aphids. In the populations single head reared on wheat in Petri dishes, the proportion of red aphids also increased as temperature was increased and stabilized at constant proportions of red aphids and green aphids. At 31 ℃, all green aphids died so that the population stabilized with 100% of the red aphids. As temperature was increased, the minimum time for population increase of the green aphids initially became shorter before becoming prolonged. In contrast, the minimum time for population increase of the red aphids was progressively more prolonged with each temperature increase. The most rapid rate of population increase for the green aphids was observed at 26 ℃. The highest temperature employed (31 ℃) significantly inhibited population increase of both the green and the red aphids but the effect was greater for the green aphids. The mean generation time of the green aphids exhibited a similar pattern with increasing temperature; i.e., prolongation after an initial reduction. In contrast, the mean generation time of the red aphids steadily increased with increasing temperature. Thus, the growth of red aphids is able to be promoted by modestly high temperatures but is inhibited at very high temperatures. In conclusion, we have demonstrated that the body-color of the grain aphid Sitobion avenae depends on temperature with the red biotype being induced at higher temperatures. In the constant temperature treatments, the initial transformation from green to red occurred between 21 ℃ and 23 ℃. The temperature at which 50% of individuals of Sitobion avenae posterity are the red biotype and 50% are green (T₅₀) was estimated to be 25.91 ℃. Very high temperatures significantly inhibit the growth of both biotypes. These observations provide basic information that may help understanding of the ecology, genetics, and evolution of aphid populations.