Sympatric races of pea aphids on alfalfa and red clover are highly ecologically specialized and significantly reproductively isolated. Much of the restriction of gene flow between the specialized populations is due to habitat choice behavior of the winged colonizers (Via 1999). Here, we document additional pre- and postmating reproductive isolation through selection against migrants and hybrids in the parental environments. First, a group of randomly chosen genotypes from each race that were experimentally migrated between hosts had very low survival and reproduction on the alternate host relative to genotypes originating from that host (natives). Such selection against cross-host migrants forms a premating barrier to gene flow because it is likely to reduce migrant frequencies before the sexual forms are induced in the fall. Our reciprocal transplant experiment also shows that natural selection acts directly on individual migrants between the crops to favor host choice behavior: genotypes from each host suffered large losses of fitness when forced to migrate to the alternate host plant relative to the fitness they would have enjoyed had they been able to choose their native host. In a companion field study, sequential sampling throughout the summer in newly colonized fields of both alfalfa and clover revealed a decrease in the frequency of host-specific marker alleles characteristic of the alternate crop. These field data further support the hypothesis that selection disfavors migrants that cross between crops. Second, when two sets of F1 hybrids between the races were reciprocally tested on alfalfa and clover, both sets had significantly lower average fitness than the specialized parent in each of the two environments. This demographic selection against hybrids in the parental environments is a source of postmating reproductive isolation between the specialized races. Finally, significant genetic variation in fitness traits was seen among F1 hybrid genotypes from both crosses between alfalfa and clover specialists. Although this variation suggests that a generalized pea aphid could evolve, such generalists are not seen in field collections of these populations.