Larvae of the Carolina sawyer Monochamus carolinensis (Olivier) (Cerambycidae) and bark beetle larvae (Scolytidae) often simultaneously feed in phloem of recently killed pine trees. Our investigations reveal that M. carolinensis larvae may act as facultative intraguild predators of bark beetle larvae. Phloem sandwiches were used in four experiments to examine inter- and intraspecific interactions. We discovered that all sizes of M. carolinensis larvae killed bark beetle larvae. Seventy-six percent of the killed bark beetle larvae were consumed by M. carolinensis, including 58% that were entirely ingested. Cannibalism in M. carolinensis occurred in every experimental trial. Based on this evidence, M. carolinensis, and possibly related cerambycid species associated with bark beetles, are facultative intraguild predators of larvae of other phloem inhabiting species. The consequences of this behavior may have important implications for bark beetle population dynamics.