An introduced plantation tree species, Acacia mangium Willd., is becoming invasive in the Brunei region of Borneo. To examine its invasive potential, a greenhouse, additive series experiment (target-neighbour) involving seedlings of A. mangium and those of a common native heath-forest (kerangas), Melastoma beccarianum Cogn. was carried out under low and high light regimes in intra-and interspecific combinations over a 6-month period. Significant variations in growth parameters (other than biomass allocation patterns) existed amongst seedlings from different treatments. A major part of this variation in growth could be attributed to the main factors of target species, neighbour species, and competition (seedling density). For the growth variables examined, the target-species response was not consistent across light regimes. Under high light conditions, Acacia was the better competitor; the LotkaVolterra competition coefficient effect of Melastoma on Acacia was lower (a=0.30) than the effect of Acacia on Melastoma (b=0.54). However, the reverse occurred under low light conditions with Melastoma gaining the upper hand (a=1.45 and b=0.44). These results show that light (and hence disturbance) can strongly influence the pattern and intensity of both intra-and interspecific competition between invasive and local flora species. Relatively intact forest is unlikely to be invaded by Acacia trees (as they are poor competitors under this scenario). On the other hand, the Acacia trees can easily invade disturbed forests, especially those prone to recurring drought and fire, and over time convert the habitats to nearly monospecific stands, as is presently being observed in Brunei.