The Russian Wheat Aphid (RWA), Diuraphis noxia (Mordvilko), is an invasive species attacking and destroying wheat and barley crops in the US. RWA was introduced into Mexico in 1980 and migrated to Texas by 1986. RWA rapidly spread to California and into Canada by 1988. After 1994, populations decreased in most states, however, RWA remains a problem in the Great Plains. The introduction, spread, and shrinking of RWA populations offers a unique opportunity to study genetic variation of invasive species. Today, RWA populations appear genetically homogeneous. No variation was found in cytochrome oxidase I (COI) mtDNA sequences between RWA collected after 2002 (n=40). However, 1.8% – 8.3% nucleotide divergence in COI sequences was found between extant populations and 4 individuals collected between 1989 and 1995, suggesting genetic variation decreased dramatically since RWA’s initial introduction. To quantify changes in temporal and spatial variation of RWA populations over 20 years time, we are sequencing the COI from RWA collected in WY, CO, NE, KS, OK, NM and TX during 1986 to 2006. Besides mtDNA sequences, microsatellite DNA markers are being used. The completed study will describe the population genetics of RWA since its first introduction to present day.