A study was conducted to determine the instantaneous growth rate of freshwater Nannochloropsis spp. at five different salinity levels (T1, 0 ppt - control; T2, 5 ppt; T3, 10 ppt; T 4, 15 ppt; and T5, 20 ppt) and were arranged in a Complete Randomized Design (CRD). Nannochloropsis spp. was cultured in fifteen 500 ml plastic experimental bottles based on the assigned treatment for eleven days. Initial sampling was done during the start of the experiment, and everyday thereafter. Cell density was counted using haemacytometer and viewed under the microscope. Results showed that the highest cell density and instantaneous growth rate of freshwater Nannochloropsis spp. was obtained in Treatment 3 with 18,510,000.00 cells/ml and16.00±0.36 cells/ml, respectively, while Treatment 5 had the lowest cell density and growth rate (10,040,833.33 cells/ml; 15.38±0.46 cells/ml).One-way ANOVA and Duncan’s Multiple Range Test revealed there was a significant difference (p<0.05) on the days of culture (days 3, 7 and 8) among treatments. Results for both cell density (cells/ml) and instantaneous growth rate (K) proved that freshwater Nannochloropsis spp. can tolerate a wider range of salinity from 5 ppt to 15 ppt. However, further studies should be conducted at a wider range of salinity levels and other variables such as nitrogen levels and light intensity to have a complete picture of the reproductive biology of this species. Keywords - Aquatic Ecology, instantaneous growth rate, freshwater Nannochloropsis spp., salinity levels, experimental design, Philippines, Asia