A field sampling survey of longhorned beetles (Coleoptera: Cerambycidae) on Hydrangea arborescens L. and Daucus carota L. was undertaken at LaRue-Pine Hills Research Natural Area, Union Co., Illinois in 1987–1988. Sixteen species (10 genera) were collected, with the lepturine genera Typocerus LeConte and Strangalia Serville being the most common. Three species, Typocerus velutinus (Olivier), Strangalia luteicornis (F.), and Euderces pini Olivier, were considered dominant; they comprised 79.8% of the total adults collected. Six species were observed in copula, with most (94.9%) on H. arborescens and in shade near an oak-hickory forest. New adult host plant records were E. pini on H. arborescens, and Elaphidion mucronatum (Say) and Neoclytus mucronatus (F.) on D. carota. Collections of cerambycids on white flowers of H. arborescens in shade close to larval host sites (an oak-hickory forest) probably accounted for its greater diversity of cerambycid species.