The goals of this study were to document natural hybridization between Rhododendron periclymenoides and R. atlanticum and to assess its significance relative to herbivory by Pyrrhalta rufosanguinea, a leaf beetle believed to be monophagous for R. periclymenoides. Populations of the Rhododendron species and a putative hybrid swarm were compared using a mixture of quantitative morphological and micro-molecular characters. Herbivory in parental and hybrid populations was estimated in cafeteria style host-choice experiments. Principal components analysis and discriminant analysis suggested bidirectional introgression in the hybrid population. Feeding levels were significantly higher for putative introgressants to R. periclymenoides than for introgressants to R. atlanticum. In the hybrid population, percent leaf area eaten by the beetles was positively correlated with leaf size and concentrations of the flavonoid asebotin and its aglycone, but negatively correlated with glandular trichome density and the concentration of myr- icentin 3-0-rhamnoside. Two hypotheses regarding the evolutionary significance of natural hybridization between the two Rhododendron species are proposed: 1) introgression from R. atlanticum to R. periclymenoides may result in greater resistance of R. periclymenoides to P. rufosanguinea; 2) gene flow from R. periclymenoides to R. atlanticum may allow P. rufosanguinea to become adapted to R. atlanticum.