The cerambycid beetle, Phoracantha semipunctata F., was introduced into California in the mid-1980s and killed large numbers of Eucalyptus host trees. The populations of the borer declined to very low levels in the mid-1990s following the establishment of the congener, Phoracantha recurva Newman, and the intentional introduction of the egg parasitoid, Avetianella longoi Siscaro. The distributions of the beetles overlap in the Australian native range, but one species has replaced the other in the adventive range in California. One possible explanation is differential susceptibility to natural enemies introduced for biological control. An alternative explanation for the reduced abundance of P. semipunctata is asymmetric interspecific competition between the two species. To test this hypothesis, equal larval densities of each species were introduced into host logs. In all cases, more P. recurva adults emerged than P. semipunctata adults, but the presence of congeners did not have a different effect than the presence of an equal density of conspecific individuals. Neither the temporal order of introduction or bark thickness altered the outcome of potential competitive interactions. Consequently, it appears that the ecological replacement of one borer with another in the adventive environment in southern California may not be a result of bottom-up intraguild competitive interactions. The top-down effects of natural enemies on P. semipunctata have most likely led to its decline.