The NAWQA Program is studying mercury cycling in urban and nonurban watersheds across the Nation to build an understanding of how natural features and human activities affect the transformation, transport, and bioaccumulation of mercury in stream ecosystems. As of August 2008, mercury was the second leading cause of stream impairment throughout the Nation (USEPA 303(d) list). Methylmercury is a neurotoxin that is biomagnified in aquatic food webs so that piscivorous fish and wildlife, and humans that consume fish, are potentially at greater risk of exposure to methylmercury. USGS data and research may aid in the development of more rigorous models that relate water quality to mercury bioaccumulation, thereby enhancing capabilities for predicting mercury contamination in fish.