Ipomopsis aggregata and I. tenuituba are two sister species that frequently produce natural hybrids. Bischoff et al. study floral scent in parental species and natural hybrids to explore potential scent mediation of prezygotic and postzygotic reproductive isolation. They find that there is qualitative overlap in the floral bouquets, yet the quantitative emission rates differ. A striking difference is the compound indole, emitted almost exclusively by I. tenuituba at night. Emissions by hybrids are highly variable, and not generally intermediate between the parents, although indole emissions are intermediate. The indole signal could potentially serve as a hawkmoth attractant that mediates reproductive isolation both before and after hybrid formation.