There are at least three genetically different forms of Sitobion on cereal crops in southern Australia. These forms are morphologically similar but can be distinguished by protein electrophoresis. We believe that two species are represented: a species near S. fragariae (Walker), with a diploid chromosome number of 18, and two chromosomal races of S. miscanthi (Takahashi), one with 2n = 17 and the other with 2n = 18. These three forms are common in the field at different times of the year. Experiments were carried out to determine whether this seasonal difference between the three genetic forms reflected different environmental tolerances. Growth rate, reproductive rate and offspring weight were used as parameters of performance under different temperature regimes. The results showed that the observed seasonal differences reflect different temperature tolerances.