As global warming presents an increasing threat to Earth’s ecosystems, species will be forced to acclimate or adapt to a warmer climate. However, it is unlikely that all species will respond the same to warming or have similar capacity for thermal acclimation and adaptation. Species that live close to or at their thermal optimum may have a physiological advantage now, but can be outperformed by species that occur below their optimum if both are exposed to warmer temperatures. Furthermore, it is unknown how physiological acclimation impacts ecological interactions like competition for food, shelter, and dominate rank. The extent that species can adapt to temperature is critical in determining their individual performance and outcome of ecological interaction in a warmer environment. Here I report a behavioural study using two coral reef damselfish, Pomacentrus amboinensis, and Pomacentrus moluccensis, raised in elevated temperatures then placed in competition arenas to determine whether developmental thermal acclimation affects competition for shelter resource. After a 90 day acclimation period, across and within species combinations at three temperatures were performed and competitive interactions were quantified. Preliminary results support that aggressive interactions for P. amboinesis tended to decrease as temperature rose while P. moluccensis tended to increase competitive interactions with increasing temperature. There was a significant reduction in competitive interactions for all treatments acclimated to temperature compared to non-acclimated fish. Though P. amboinensis tends to be dominant over P. moluccensis in current temperatures, these results show how this relation may invert when exposed to warmer temperatures. This, and similar studies, give insight to the mechanisms that will drive community composition in a future warmer world.