The aphid Aphis gossypii Glover generally is seen as a highly polyphagous species with mainly parthenogenetic reproduction. This study shows that genotypes living on cucumber and chrysanthemum behave as genetically distinct host races, Little or no reproduction occurs after reciprocal host transfers. Rearing for three generations of the cucumber clones on chrysanthemum did not lead to a better performance measured as developmental time and adult weight at the day of first reproduction. Only larval mortality decreased over these three generations. Thus, performance is not likely to improve through induction. Further, the chrysanthemum clones of A. gossypii were observed to produce sexual females and males under short day conditions. These results show that A. gossypii developed distinct host races with different abilities of colonizing host plants and with more potentials of sexual reproduction, and consequently evolutionary differentiation, than previously regarded.