Culture experiments were conducted with the alga Scenedesmus acutus and hetero-trophic bacteria to examine if the nature of their relationship changes according to the balance of light and nutrient supplies. Mixtures of algae and bacteria were grown in vanous combinations of 6 light intensities and 4 phosphorus (P) concentrations at high N:P ratio (80:l). We used an artificial medium composed of inorganic nutrients so that bacteria relied on organic matter released by algae as carbon (C) source. Every 2 d, 25 % of the culture suspension was replaced by fresh medium. At the end of incu-bation when both bacterial and algal densities were stabilized, bacteria were separated from algae. Bioassays with glucose and/or inorganic P enrichment were then performed to assess the extent to which bacterial growth rate was limited by organic C or inorganic P. The algal density in the semi-batch culture was low under the light intensity 1 5 5 FE rn-'s-' regardless of P concentrations, while it was higher at higher light and P supply rate above that light intensity. The bacterial density was higher in the cultures where algal density was higher. The bioassay revealed that bacteria were C limited at the light intensity <55 pE m-2 S-', indicating a commensal relationship between algae and bacteria. Above that light intensity, bacteria suffered from deficiency of organic carbon rather than P at lowest P supply rate, because of low algal biomass due to a shortage in P supply. At moderate P supply rates and light intensities 255 PE m-2 S-', however, bacterial growth was limited by P rather than organic C, because supply of organic C from algae exceeded P supply relative to bacterial demand. Further increase in P supply released both algae and bacteria from P limitation. Thus, con~petitive interaction for P was most intense at a moderate P supply rate. These results demonstrate that there is a shift between comrnensalism for C and competition for P depending on light intensity and nutnent supply rate.