1. Growth of seedlings of 15 rain-forest tree species was compared under controlled conditions, at six different light levels (3, 6, 12, 25, 50 and 100aylight). 2. Most plant variables showed strong ontogenetic changes; they were highly dependent on the biomass of the plant. 3. Growth rate was highest at intermediate light levels (25-50€above which it declined. Most plant variables showed a curvilinear response to irradiance, with the largest changes at the lowest light levels. 4. There was a consistent ranking in growth between species; species that were fast growing in a low-light environment were also fast growing in a high-light environment. 5. At low light, interspecific variation in relative growth rate was determined mainly by differences in a morphological trait, the leaf area ratio (LAR), whereas at high light it was determined mainly by differences in a physiological trait, the net assimilation rate (NAR). 6. NAR became a stronger determinant of growth than LAR in more than 10-15aylight. As light availability in the forest is generally much lower than this threshold level, it follows that interspecific variation in growth in a forest environment is mainly owing to variation in morphology.