Within species of polyphagous herbivores, individuals may prefer or have greater fitness on one host than on others within the species range. Such intraspecific specialization can be caused either by genetic differences or by the effects of experience (conditioning) on a given host plant. Separating the causes of intraspecific variation in host use in an important issue in understanding the potential for host shifts in herbivorous species. In the pea aphid, a polyphagous pest of forage crops, various clones collected from a local area have been shown to be specialized such that they have higher fitness on either alfalfa or red clover. The experiment reported here was designed to test whether this observed specialization could be significantly altered by prolonged experienced on the alternate plant species. In a factorial design, replicate lineages of two specialized clones were maintained on both alfalfa and red clover for three generations; each line was then tested on both crops. Experience on the alternate crop had no significant effect on the relative performance of these two specialized clones. These results support the suggestion that differences in host plant performance among pea aphid clones are genetically based. They also suggest that individuals that move between crops are likely to be selected against, since they appear to be unable to significantly improve in performance through time as a result of experience. Such selective elimination of intercrop migrants will promote genetic divergence of nearby subpopulations on different crops.