The effects of low nitrogen-phosphorus ratios on microalgae from a large eutrophic freshwater lake in the Philippines were investigated. Natural microalgal populations from Laguna de Bay, the largest lake in the Philippines, were cultured using three different nitrogen-phosphorus weight ratios (2N:1P; 6N:1P and 12N:1P) at two phosphorus concentrations (0.25 and 0.5 mg l^–1) in each case. The growth and genera composition of the cultures under the different treatments were followed for a 12-week period. Community level responses were assessed based on species richness (s), Shannon-Wiener Index (H’), Simpson Index (λ) and Evenness (J’). Among the different microalgal groups, only the chlorophytes showed a significantly higher density in response to the 12N:1P treatment at the higher P concentration, indicating that the nutrient ratio had a significant interaction with the nutrient levels used in the experiments. The genera found in the different treatments were generally similar; however, the degree of dominance of some varied with treatment during the experiment. The succession of dominant genera also differed among the N:P treatments. The diatoms like Fragilaria, Aulacoseira (= Melosira) and Nitzschia dominated the lowest N:P. On the other hand, chlorophytes (Kirchneriella and Scenedesmus) dominated the highest N:P treatment, particularly from the second to the seventh week of the experiments with the diatoms becoming co-dominant only towards the eighth week until the end of the experimental. The 6N:1P treatment showed a mixed dominance between the diatoms and the chlorophyte genera. The various indices of diversity indicate significantly lower diversity only in the 12N:1P at 0.5 mg l^–1 P and not in 12N:1P at 0.25 mg l^–1 P.