Cyanobacteria or blue-green algae is a major group of microorganisms containing chlorophyll and phycocyanin pigments. The genus Synechococcus is an example of cyanobacteria. Research on comparison of the growth curve of Synechococcus HS-7 and Synechococcus HS-9 were grown at temperature 35°C and initial pH of medium 9 had been performed. Synechococcus used in the study is Indonesian indigenous microorganisms. Synechococcus HS-7 isolated from Ciseeng hot springs in West Java with water temperature between habitats was 43°C and pH ranges were 7.5 - 8. Synechococcus HS-9 isolated from Rawa Danau Banten hot springs with water temperature of habitats was 50°C and pH ranges were 6.5 - 7.5. The purpose of this research is to know the differences of the growth curve of Synechococcus HS-7 and Synechococcus HS-9 with the mean of cell density parameter. Counting cell density was done for 7 days from day-0 (t0) to day-7 (t7). Each treatment carried out in two replications. The result showed that there were differences between cell density of Synechococcus HS-7 and Synechococcus HS-9. All of Synechococcus strains experienced adaptation (lag) phase and exponential (log) phase. Synechococcus HS-7 experienced adaptation (lag) phase shortly than Synechococcus HS-9. Adaptation (lag) phase in Synechococcus HS-7 occurred at t0 - t1 while in Synechococcus HS-9 occurred at t0 - t3. Exponential (log) phase in Synechococcus HS-7 also occurred faster at t1 - t4. Synechococcus HS-9 experienced exponential (log) phase from day-3 (t3) until the last day of observation. The highest cell density of Synechococcus HS-7 occurred at t6 while Synechococcus HS-9 occurred at t7 for 7 days observation. Synechococcus HS-9 grows well in CT medium with initial pH 9 than Synechococcus HS-7.