The diatom flora of a core taken from the region of the central buoy in Esthwaite has been analysed quantitatively and qualitatively. The diatom flora of the sediments laid down from the early zones of the Late-glacial to the present day can be subdivided into five periods characterized by indicator species and variations in diatom productivity. The zones in Esthwaite have been compared with those from Windermere and Kentmere and shown to agree in general features, indicating a similar historical development for all three lake basins. The diatom periods show good correlation with the zoning derived from pollen analysis; and the change at the Boreal-Atlantic Transition from base-rich to base-poor status, which was first detected in Kentmere, has been substantiated in Esthwaite. Comparison with the present-day diatom flora suggests that a transition to eutrophic conditions, detected in late Post-glacial time, is still occurring.