The aphid Rhopalosiphum maidis is a serious pest against cereal plants. In Egypt, it causes serious damage to many cereal plants. The principal idea of this study was achieved based on the observation of various seasonal invasion rates of this pest on different host plants suggesting climatic adaptation by R. maidis polymorphs. The invasion rates on winter plants were higher than those of summer. Morphologically, all collections were belonging to R. maidis. The authors assumed that different biotypes of R. maidis are present in the studied area. To address this hypothesis, the mitochondrial cytochrome oxidase I (COI) gene was amplified in aphids collected from wheat, barley, sorghum and maize in Assiut, Egypt. Polymerase chain reactions (PCR) identified 687 bps products corresponding to a partial sequence of (COI). Multiple sequence alignment of the sequenced products revealed that at least five haplotypes are present on the tested plants. Phylogenetic analysis using the neighbor-joining method identified five clades that represent different biotypes. The identified clades contained mixtures between summer and winter R. maidis collections suggesting that different biotypes are present in mixtures on the same plant. Our findings showed that R. maidis develops different polymorphs on various host plants at different climatic conditions.