Abstract.1. Host plant preference experiments were conducted with closely related taxa of the aphid genusCryptomyzus.Males, and presexual morphs (sexuparae and gynoparae), were used to determine the impact of host plant choice on reproductive isolation. In the case of host‐alternating species these morphs are migratory and so will select the host plant.2. Host plant preference of two closely related taxa ofC. alboapicalis(Theobald) was found to promote their reproductive isolation. The preference of sexuparae of these monoecious taxa was more pronounced than that of the males.3. Host plant preference and subsequent production of oviparae showed thatC. galeopsidis(Kaltenbach) consists of two host races restricted toRibes rubrumL. andR. nigrumL., respectively. The existence of clones, intermediate in their preference and reproductive performance on these plants, suggests that hybridization occur