ABSTRACT: Diel variations in the protozoan grazing and viral-mediated mortality of Synechococcus spp. were simultaneously estimated using a modified dilution method in the coastal waters off Taiwan during 4 cruises in summer 2011. During the study period, the abundance of Synechococcus spp. showed a clear diel cycle and ranged from 1.2 × 104 cells ml−1 at 08:00 h to 9.8 × 104 cells ml−1 at midnight. The frequency of dividing cell (FDC) values were highest between afternoon and dusk and decreased to <10% between 20:00 and 08:00 h. Synechococcus spp. specific growth rates varied during the experimental period from 0.025 to 0.033 h−1 and 0.050 to 0.085 h−1 at daytime and nighttime, respectively. Rates of Synechococcus spp. mortality due to nanoflagellate grazing averaged 0.031 h−1 during daytime and 0.041 h−1 at night. The estimated mortality due to viral lysis was between 0.011 and 0.019 h−1 during daytime and between 0.026 and 0.065 h−1 at night. Nanoflagellate grazing was the dominant cause of Synechococcus spp. mortality during daytime; however, mortality from viral lysis and nanoflagellate grazing was more balanced at night.