ABSTRACT: Viruses have been implicated as major players in aquatic nutrient cycling, yet few data exist to quantify their significance. To determine the effect of viruses on ammonium regeneration by bacteria, experiments were carried out in the oligotrophic Indian Ocean and productive False Creek, Vancouver, Canada. Bacteria were concentrated and then diluted with virus-free water to reduce virus abundance, or with virus-replete water to restore natural virus abundances. Virus-replete treatments showed increased ammonium concentrations compared to treatments with viruses removed (differences of 0.287 ± 0.14 and 1.44 ± 0.73 µmol l⁻¹, mean ± SD, in the Indian Ocean and False Creek, respectively). Bacterial abundances were lower, while phytoplankton abundances and chlorophyll a (chl a) concentrations were greater in the virus-replete treatments, consistent with the increased availability of ammonium in the presence of viruses. These data demonstrate that viral lysis leads to ammonium production, likely through the liberation of dissolved organic N that is remineralised by uninfected bacteria. In turn, the released ammonium fuels primary production. These results show that viruses play a critical role in the marine N cycle, and suggest that viral lysis likely supplies a significant portion of the global N requirements of phytoplankton.