AbstractOcean viruses are abundant, ubiquitous, and play important roles in global biogeochemical cycles through mortality, horizontal gene transfer and manipulation of host metabolism. However, the ability to link viruses to their hosts in a high-throughput manner bottlenecks our ability to understand virus-host interactions in complex communities. Here, we present viral tagging (VT), a method that combines mixtures of isotope labeled host cells and fluorescent viruses with flow cytometry. In a single experiment, we can screen 107 uncultivated ocean viruses with a single strain of Synechococcus. These viruses can then be sequenced to quantitatively link objectivelydefined environmental viral populations, and their genomes, to their hosts.