The flea beetlePhyllotreta cruciferae(Goeze) is selective of its host plants, both between cultivars and between plants of different ages within cultivars. Previous work has associated these preferences with chemical differences between plants. Factors suspected of affectingP. cruciferaebehavior, including host plant chemistry and morphology, were isolated and examined in laboratory assays.P. cruciferaefed equally well on collard and kale cotyledons in both choice and no-choice assays. During 15-min observation periods, flea beetles spent more time on cotyledons than on older leaves of either cultivar. Injured collard and kale cotyledons were fed upon more than uninjured cotyledons in both choice and no-choice assays, and this preference is associated with release of chemical cues from the host. Leaf morphology, however, also affected flea beetle behavior. During 15-min observation periods,P. cruciferaespent more time on 6-wk-old collard leaves than on 6-wk-old kale leaves, and on collard leaf models than on kale leaf models treated with identical leaf extracts. Laboratory results indicate thatP. cruciferaehost preference is associated with chemical and morphological differences and help explain the spatial patterns in population density observed in the field.