Ambient ultraviolet radiation (UVR) is harmful to many biological systems and increased UVR, due to a reduced ozone layer, may have many unforeseen consequences. Viruses are the most abundant biological particles in the sea and are thought to play an important role in the structure and functioning of aquatic ecosystems. Although an increasing number of studies have been published during the last 15 years, aquatic viral ecology is still in its infancy and little is known about the effect of environmental factors on virus life cycle and host–virus interactions. Using flow cytometry, we have investigated the effect of UVR (UVB intensity: 0.22 W m−2 and UVA/UVB ratio ∼30) on five different cultured marine phytoplankton host–virus systems (CeV-Chrysochromulina ericina, EhV-Emiliania huxleyi, MpV-Micromonas pusilla, PpV-Phaeocystis pouchetii and PoV-Pyramimonas orientalis). Viruses appear to be susceptible to UV, but also they might provide some protection to their hosts. It is shown that (i) some of the investigated microalgae that have been co-cultured with viruses are less sensitive (e.g. P. pouchetii, M. pusilla) to UVB stress compared to susceptible microalgae (i.e. virus-free cultures), (ii) different viruses have different sensitivities to UVB in terms of both their abundance patterns (no effect for most of them except EhV) and infectivity (from no effect for PoV, to complete inactivation for PpV), (iii) UVA has no effect on host–virus interactions. Our results show UVB to be a potentially important factor in the regulation of virus–host interactions in surface waters.