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Extensive research exists regarding the toxicity of metals (including arsenic) to aquatic invertebrates. However, there has been little consideration of potential synergies between metals and ultraviolet (UV) radiation–despite considerable debate on this topic in human health research. Ultraviolet radiation is nearly ubiquitous in the natural environment, but it is generally overlooked as a confounding variable in toxicological assessments. We evaluate synergies between arsenic and solar UV radiation using the crustacean, Ceriodaphnia dubia. Both laboratory (with simulated solar radiation) and outdoor (with natural solar radiation) factorial experiments were performed with two intensities of UV (low and high) and four arsenic concentrations (0, 1, 1.25 and 1.5mg/l). The laboratory experiment was multigenerational, examining survival and fecundity effects. The combination of high UV+1.5mg/l As adversely impacted survival; whereas, High UV+0mg/l As and Low UV+1.5mg/l As treatments did not. These results suggest synergism. This pattern was consistent for all three generations. Fecundity effects were not consistent across generations, and arsenic was demonstrated to have a greater impact than UV. Outdoor experiments were limited to assessing survival. Exposures in September 1999 resulted in a pattern similar to that in the laboratory exposure. High UV+1.5mg/l As treatment elicited diminished survival as compared to high UV+0mg/l As and low UV+1.5mg/l As. These results indicate that a synergistic effect between arsenic and UV exposure is possible under ambient conditions and within a relatively narrow dose range. The mechanism of this effect is unknown but could include synergistic genotoxic or oxidative stress. These findings point to the importance of using realistic UV exposures when determining criteria for protection of aquatic life.