Desmids are exclusively-freshwater green algae (Chlorophyta) which, because of their mode of sexual reproduction by conjugation, are grouped with the well-known filamentous alga Spirogyra in the Class Zygnemaphyceae (previously the Class Conjugatophyceae (Round 1971)). This key is intended to make possible the identification of the most commonly encountered species of the English Lake District. Which species they are has been determined by the examination of collections from the open waters and weedy margins of lakes, and from smaller, mainly upland, bodies of water known locally as tarns; also from shallow, peaty pools. Some of the tarns are in rocky basins with a sparse macrophytic vegetation while others from peaty areas often contain considerable amounts of Sphagnum. Only those species or varieties found in 3 or more of the tarns and lakes have been included in the key.