Few quantitative data exist on human virus inactivation by sunlight and the relationship between human and indicator viruses under sunlit conditions. We investigated the effects of sunlight on human viruses (adenovirus type 2, poliovirus type 3) and bacteriophages (MS2, Q-Beta SP, Fi, M13, PRD1, Phi-X174, and coliphages isolated from Avalon Bay, California). Viruses were inoculated into phosphate buffered saline or seawater, exposed to a laboratory solar simulator for ≤12 h, and enumerated by double agar layer or cell culture to derive first-order inactivation rate constants (k(obs), h(-1)). The viruses most resistant to sunlight were adenovirus type 2 (k(obs)= 0.59 ± 0.04 h(-1)) and bacteriophage MS2 (k(obs)= 0.43 ± 0.02 h(-1)), which suggests MS2 may be a conservative indicator for sunlight resistant human viruses in clear water when sunlight inactivation is the main removal mechanism. Reasonable agreement was observed between somatic coliphage inactivation rates measured in the solar simulator (k(mean) = 1.81 h(-1)) and somatic coliphages measured in the surf zone during a field campaign at Avalon Bay during similar sunlight intensity (k = 0.75 h(-1) at log-RMSE minimum; k(range) = 0.54 h(-1) to >1.88 h(-1); Boehm, A. B. et al. Environ. Sci. Technol. 2009, 43, (21), 8046-8052). Hence, measuring sunlight inactivation rates of viruses in the laboratory can be used to estimate inactivation in the environment under similar sunlight and water quality conditions.