Changes in fitness parameters as a function of colony size (one versus 10 aphids) were measured in two biotypes (RWA1 and RWA2) of the Russian wheat aphid, Diuraphis noxia (Mordvilko) (Homoptera: Aphididae), feeding on three cultivars of wheat, Triticum aestivum L., at two temperatures. ‘Trego’ is a cultivar with no specific resistance to D. noxia, whereas, ‘Stanton’ and ‘Halt’ express Dny and Dn4 resistance sources, respectively. Feeding in a group accelerated the development of RWA1 on Trego and Stanton at 20°C, but not at 24°C, whereas grouped RWA2 developed faster than solitary RWA2 on all three cultivars at 24°C, but not at 20°C. Survival (first instar–adult) of RWA2 also was improved by grouping on Stanton and Halt at 24°C, but solitary RWA2 survived better at 20°C on all three cultivars. The reproductive rate of RWA1 was improved by grouping on Trego and Stanton at both temperatures, but only on Halt at 24°C. Lifetime fecundity of RWA1 also was increased by grouping in all cases except for Trego at 20°C. Grouped development increased the reproductive rate of RWA2 on all three cultivars at 24°C, but had no effect at 20°C. Grouped RWA2 developed and reproduced faster than grouped RWA1 on all three cultivars at 24°C. Thus, the fitness of D. noxia was positively correlated with group size during colony establishment, but the effects were sensitive to temperature, being more pronounced at 20°C for RWA1 and at 24°C for RWA2.