Aims: To investigate norovirus (NoV) and F-specific RNA (FRNA) bacteriophage inactivation in seawater under simulated sunlight and temperature conditions representative of summer (235Wm(-2); 17 degrees C) and winter (56Wm(-2); 10 degrees C) conditions in Ireland.Methods and Results: Inactivation experiments were carried out using a collimated beam of simulated sunlight and 100ml of filtered seawater seeded with virus under controlled temperature conditions. NoV concentrations were determined using RT-qPCR, and FRNA bacteriophage concentrations were determined using RT-qPCR and by plaque assay. For all virus types, the fluence required to achieve a 90% reduction in detectable viruses (S-90 value) using RT-qPCR was not significantly different between summer and winter conditions. S-90 values for FRNA bacteriophage determined by plaque assay were significantly less than those determined by RT-qPCR. Unlike S-90 values determined by RT-qPCR, a significant difference existed between summer and winter S-90 values for infectious FRNA bacteriophage.Conclusions: This study demonstrated that RT-qPCR significantly overestimates the survival of infectious virus and is therefore unsuitable for determining the inactivation rates of viruses in seawater.Significance and Impact of the Study: Results from this study provide initial data on the inactivation of NoV and FRNA bacteriophage in seawater under representative summer and winter conditions and will be of interest to shellfish and water management agencies alike.