The intergenic spacer region of the rRNA cistron was used as a molecular fingerprinting probe to study clonal diversity in Schizaphis graminum (Rondani). A high degree of clonal diversity was found in overwintering populations on wheat from Kansas, Texas and Oklahoma. Out of 184 individuals tested, 132 unique maternal lineages wereidentified. Overall, clonal diversity of overwintering populations was no less than in spring and summer populations. However, in some fields there were more instances where individuals of the same genotype were identified. By partitioning the total population diversity into various sampling components (fields, counties and states), 93.1% of the total diversity was found among individual S. graminum within fields. Overwintering of many genetically distinct clones as parthenogenetic morphs is one mechanism by which clonal diversity is maintained in populations of S. graminum. The incidence of clonal diversity further substantiates the importance of the S. graminum holocycle for generating genetic heterogeneity in the USA.