Abstract The presence of heptachlor epoxide in harbour seals ( Phoca vitulina) and white-beaked dolphins ( Lagenorhynchus albirostris) has been confirmed by chemical derivatization and mass spectrometric analyses. An unknown compound at a retention time close to that of heptachlor epoxide interfered in the determination by electron capture detector—gas chromatography. On a WCOT CP-Sil 7 column programmed from 85 to 175°C two poorly separated peaks could be detected. Heptachlor epoxide contents up to 1.7 mg/kg (on a fat basis) were obtained, which were about 200 times lower than the PCB levels. The same ratio was also observed in fish from the same area.