Coptotermes gestroi (Wasmann), an invasive termite species in Taiwan, has been a major structural pest in southwestern Taiwan. C. gestroi was recently reported to have infested living trees in a Taiwanese forest, showing its potential of becoming an invasive forest pest in Taiwan. To determine whether C. gestroi have naturalized in the forests, we monitored their dispersal flights and estimated their colony development status on the basis of their worker and soldier morphology. The results showed that mature C. gestroi colonies occurred in forest, indicating that C. gestroi has naturalized. The colony sizes of C. gestroi were estimated using a triple mark-release-recapture method. The three studied colonies contained 0.12-0.20 million individuals, which was smaller than that reported in previous studies conducted in urban environments. We speculate that C. gestroi population is suppressed by ants and another dominant termite species, Odontotermes formosanus (Shiraki). Colony elimination was achieved four months after employing termite baits for controlling the three colonies of C. gestroi in forest. In summary, although C. gestroi have naturalized in Taiwan, their further expansion in the forest has likely been restricted by ants and other termite species. Termite baits can be a practical option for controlling C. gestroi in the forests.