The invasive vine Celastrus orbiculatus Thunb. (Oriental bittersweet) dominates gap and edge environments, but may also colonize undisturbed forest. We compared survival and growth of C. orbiculatus seedlings in field plots under 2%, 28% and 100% sun. From transplanting through the first autumn, survival and growth did not differ among treatments. In the second growing season, survival at 2% sun was 76%, compared to 96% in 28% sun. Growth and biomass were greater in the 100% and 28% sun treatments than 2% sun. The ratio of leaf to total biomass (LBR) decreased with shade, but leaf mass per leaf area (LMA) decreased proportionally more, so that the leaf area per unit biomass (LAR) increased in the shade. Photosynthesis, stomatal conductance and the ratio of photosynthesis to conductance (A/g) decreased in the shade. The ability of C. orbiculatus to survive under deep shade despite its slow growth implies that intact forests are vulnerable to invasion and that established understory populations should be controlled before harvesting or thinning the forest.