Viral-mediated cell lysis and microzooplankton grazing are both important sources of phytoplankton and bacteria mortality in the ocean, however, the magnitudes of these mortality sources are difficult to quantify. Using the modified (viral) dilution method, the effects of viral- and microzooplankton-mediated phytoplankton mortality can be examined simultaneously. The viral dilution method is modeled after the traditional dilution method developed by Landry and Hassett (1982), which relies on the sequential dilution of a natural community with filtered seawater to reduce the predator-prey encounter rate, thereby allowing one to simultaneously measure phytoplankton growth and microzooplankton grazing. In the traditional dilution method, viral abundance is constant across all dilutions; therefore, mortality due to viruses is included in the phytoplankton growth term. In the viral dilution method, viral abundance is diluted using 30-kDa filtered seawater. A series of these viral dilutions are run in parallel with a traditional dilution series (0.1-μm filtrate). By a simple subtraction of the measured mortality in the viral dilution from the mortality in the traditional dilution, the mortality of phytoplankton due to viruses can be quantified. Six viral dilution experiments were performed during a cruise in September/October 2008 in the California Current Ecosystem to assess the relative importance of viral- vs. microzooplankton-mediated mortality on the phytoplankton and bacterioplankton communities in this coastal upwelling system. The magnitude of microzooplankton grazing and viral-mediated cell lysis varies among different groups of phytoplankton as well as with environmental conditions.