Abstract: Recent development of virulent (Diuraphis noxia (Kurdjumov) (Homoptera: Aphididae) (RWA) biotypes is a challenge to insect resistance breeding programs. The aim of this study was to detect the existence of biotypes within two Russian wheat aphid populations sampled from Njoro and Timau locations of Kenya. Ten clones were raised from each population and used in population increase studies, virulence tests on different wheat cultivars and Amplified Fragment Length Polymorphic (AFLP) DNA fingerprinting assays. Results of two population increase tests indicated that aphid survivorship was high on Njoro biotype with mortality being concentrated at the end of the maximum life span. Timau biotype had low survivorship with mortality concentrated early in the cycle. Timau biotype had fewer progeny and lower estimates of intrinsic rate of natural increase on susceptible bread-wheats. Results of virulence tests showed that no clear virulence trend was recorded on the biotypes but Njoro biotype was appreciably more virulent on some resistant wheats than Timau biotype. One AFLP primer pair, E-AAC/M-CAC, gave amplified and polymorphic DNA fingerprints and more than one RWA genotype were identified among clones of Njoro and Timau biotypes. The study shows the existence of more than one biotype and/or genotype present within tropical RWA populations.