The Russian wheat aphid (RWA), Diuraphis noxia (Mordvilko), is a pest on wheat and barley in the Western U.S. 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 RWA2, while the extant population was designated RWA1. 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 RWA1 and RWA2 biotype distinctions, and also three additional biotypes (RWA3-Texas, RWA4-Texas, RWA5-Wyoming) determined according to plant response (Burd et al. Submitted). No sequence variation was found within or between biotypes in a 440 bp fragment of the cytochrome oxidase I gene in the mtDNA. Using random amplified polymorphic DNA (RAPD), a single band polymorphism distinguishing RWA5 was found after screening each sample with 56 primers. Microsatellite DNA markers are being used to further evaluate clonal diversity of the RWA, and variation was found distinguishing RWA biotypes. If polymorphisms are detected, inferences can be made as to which RWA clones arose from populations outside the U.S. If variation is greater between than within biotypes, it is likely they are reproductively isolated, with little or no gene flow. If variation is greater within than between biotypes, they are not reproductively isolated and gene flow occurred.