Polychlorinated terphenyls (PCTs) have been determined in aquatic biota and sediments by gas chromatography/mass spectrometry using negative chemical ionization. The use of various methods of calculation is discussed. Total PCT concentrations expressed as A (Aroclor) 5442 equivalents ranged from 0.28 to 7400 μg/kg of wet weight in biota, with hexachloroterphenyls (hexa-CTs) as the major group of CTs present. In sediments, total PCT concentrations expressed as A5442 equivalents ranged from 22 to 100 μg/kg of wet weight, with hepta-CTs as the dominant group of congeners. Total PCT concentrations in biota were approximately 1-10% of total polychlorobiphenyl (PCB) concentrations and in sediments 1025% of total PCB concentrations. The rather unexpectedly high concentrations of PCTs, especially in the sediment samples, should stimulate further research on their general environmental distribution.