[en] In the 1990 SRS Environmental Report, the statement was made that ''Since 1989, concentrations of mercury in fish collected at all locations onsite have been higher than fish collected from the Savannah River. Therefore, concentrations of mercury cannot be attributed to offsite sources.'' However, in a preliminary assessment of a subset of the mercury tissue concentration data upon which this concern was based, M. Newman of the University of Georgia, Savannah River Ecology Laboratory stated that, although there was an upward trend in the recent data for Par Pond bass, several factors precluded assignment of a cause. A study of mercury bioaccumulation in fish collected on and around the SRS was then developed to resolve some of this ambiguity. It addressed the question, ''Do fish on the SRS have significantly higher mercury concentrations than fish from offsite locations?'' The results of this study which used the mosquitofish (Gambusia holbrooki) as the sentinel species are reported herein. Criteria for optimum sampling of mercury concentrations in mosquitofish were first established in a preliminary study. These criteria were then used to accurately estimate the mercury concentrations in fish from sites on and around the SRS. The results indicated no significant differences in mercury concentrations of samples taken ''on'' versus ''off'' the SRS. However, there were significant differences in mercury concentrations of fish from locations associated with Savannah River water versus fish from locations isolated from river water input