The ferrimagnetic iron sulphide, greigite, has been extracted from freshwater sediments from Loch Lomond and Llyn Geirionydd and identified by X-ray diffraction and thermomagnetic analyses. The authigenic greigite is probably the dominant carrier of a stable natural remanent magnetization in the freshwater sediments underlying marine sediments in Loch Lomond and in the Lateglacial interstadial sediments from Llyn Geirionydd. Mineral magnetic analysis shows that greigite tends to have a higher SIRM/X ratio (approximately 70 kAm-1) than many natural magnetite assemblages. This magnetic characteristic coupled with greigite's ability to oxidize easily and to change to a less magnetic mineral upon exposure to air may be used to indicate its presence in downcore magnetic profiles. Alternating field demagnetization cannot be used to distinguish chemical from detrital remanences in Loch Lomond or Llyn Geirionydd on account of the similar magnetic stabilities of greigite and magnetite. The time lag between the acquisition of the primary detrital remanence and the secondary chemical remanence remains unknown and creates difficulties in understanding the record of paleomagnetic secular variation contained in these lake sediments. ¿ American Geophysical Union 1990