Plant macrofossil analyses of five peat cores from undisturbed fens in the flood-plain of the Ant Valley of the Norfolk Broadland show the sequence of vegetation development during the last two millennia. Macrofossil assemblages have been grouped into five regional phases and are interpreted largely in terms of the response of the vegetation to changes in sea level, climate and management. Phase 1 represents pre-Roman fen woodland communities (>2000 cal. BP); phase 2 represents salt-marsh communities formed during an estuarine phase in Romano-British times (c. 2000–1600 cal. BP); phase 3 represents ‘tussock-fen’ and carr communities suggestive of drier conditions in the post-Roman to early Medieval period (c. 1600–800 cal. BP); phase 4 represents aquatic communities indicative of wetter conditions from the late Medieval period to c. 300 cal. BP; phase 5 represents communities comparable with present-day vegetation. The biostratigraphic development of the Ant Valley floodplain mires has analogues in climatically induced humification changes of some British ombrotrophic mires, suggesting a response to similar climatic controls. Widespread human interference and control of the fen vegetation may be a relatively recent phenomenon (beginning possibly,400 cal. BP). Peat-accumulation rates in the undisturbed mire sites suggest that the original Medieval turbaries which later flooded to form the Norfolk Broads may have been at least 0.5 m shallower when dug than their present depth. The wide range of environmental conditions experienced by the mires during the last two millennia is of relevance to the development of strategies for their conservation.