This study was aimed to determine if there was any effect of chronic heat exposures and heat acclimation on the thermal thresholds and on the abundance of symbiotic bacteria of cereal aphids Rhopalosiphum padi (L.) and Sitobion avenae (F.). Aphid clones were randomly collected from the wheat fields and were reared in laboratory under controlled conditions. Thermal tolerance indices (chronic, basal and acclimated CTmax) were determined for five-day old apterous female aphids. Real time quantitative-PCR was used to assess the total eubacterial (16S) and aphid-specific bacterial symbiont gene abundance in aphids. Averagely, R. padi were more tolerant to chronic heat exposure (to 31 °C) and its CTmax values were 1.0°C higher than S. avenae. Aphid-specific symbiont genes abundance per aphid was almost similar for both of the species. Moreover, for both species, temperature-tolerant aphids exhibited significantly higher symbiont genes than the susceptible aphids. Likewise, thermal tolerance of both aphid species were found correlated with the gene abundance of total symbionts (16S), Buchnera aphidicola, Serratia symbiotica, Hamiltonella defensa, Regiella insecticola, Rickettsia spp. and Spiroplasma spp., suggesting their potential role in conferring thermal tolerance to these aphids