The longhorn beetle Tetropium fuscum F. (Coleoptera: Cerambycidae) has become established in Nova Scotia, Canada, where it coexists with Tetropium cinnamopterum Kirby. The two Tetropium species share a similar ecological niche and use the same volatile cues for mate attraction. Exotic T. fuscum was introduced near Halifax, Nova Scotia, in approximately 1990, but the rate of its spread 20 yr later has not been documented. We report a large-scale, 3-yr study that investigates the distribution of T. fuscum relative to its site of introduction. Traps baited with male-produced pheromone and host volatiles were used to estimate the relative abundance of the two Tetropium species. Adult T. fuscum emerged 1-2 wk earlier than T. cinnamopterum each year between 2008 and 2010. The spatial distribution of T. fuscum was characterized by a sharp decline in abundance in relation to its point of introduction, up to a threshold distance of approximately 80 km beyond which T. fuscum is rare in comparison with native T. cinnamopterum. The restricted range of T. fuscum 20 yr after its introduction may be attributed to limited dispersal of adults or reproductive failures of low-density populations. The distribution of T. fuscum seemed stable between 2008 and 2010. In 1 of 3 yr, the abundance of T. cinnamopterum increased with the distance to the site of introduction of T. fuscum, which suggests competitive interactions between the two Tetropium species.