Streambed-sediment samples were collected in 1992-94 at selected sites in the Connecticut, Housatonic, and Thames River Basins to determine the geographical distribution of trace elements and organic compounds and their potential for adverse biological effects on aquatic organisms. Chromium, copper, lead, mercury, nickel, zinc, chlordane, DDT, PAHs, and PCBs were detected in samples from throughout the basins, but concentrations of these constituents generally were lowest in the northern forested drainage basins and highest in the southern urbanized drainage basins of Springfield, Massachusetts, and Hartford, New Haven and Bridgeport, Connecticut. Possible anthropogenic sources of these contaminants include industrial effluent; municipal wastewater; runoff from agricultural, urban and forested areas; and atmospheric deposition. Some organic compounds pose the greatest threat to biological organisms in terms of exceedances of sediment-quality guidelines; those compounds are present at sufficiently high concentrations to potentially cause severe effects at several locations in the basins.Some trace elements represent the most geographically widespread threat to living organisms. These exceed sediment-quality guidelines over a wider geographical area, although usually by lower ratios of contaminant concentration to sediment-quality guideline than the organic compounds.