The Russian wheat aphid (RWA), Diuraphis noxia (Mordvilko), is a pest on wheat and barley in the Western States. Management is achieved primarily by resistant wheat varieties. In 2003, a biotype of the RWA caused damage to wheat with the Dn4 resistance gene in SE Colorado. The damaging biotype was designated Biotype B, while the extant population was designated Biotype A. Information on the degree of variation between and within biotypes can aid in determining the source or cause of biotype occurrence. We conducted studies to observe the variation between and within RWA Biotypes using 10 clones of Biotype A and 5 clones of Biotype B. Approximately 350 bp of the cytochrome oxidase I gene in the mtDNA was sequenced. No sequence variation was found within or between biotypes. Similarly, no variation in random amplified polymorphic DNA (RAPD) was found after screening each clone with 25 primers. Even though no molecular genetic variation was found between or within biotypes, plant response experiments showed variation within Biotype B in the degree of damage caused to wheat containing the Dn4 resistance gene. Parameters showing significant clone effects within Biotype B were chlorosis, leaf rolling, plant height, and shoot dry weight. The phenotypic variation found within Biotype B argues against the hypothesis that Biotype B populations consist of a single, genetically identical clone. Microsatellite DNA markers will be used to further evaluate clonal diversity between and within both biotypes of RWA.