Excavation of the relict stratigraphy of a drained, agriculturally reclaimed mire in Lancashire, northwest England, has revealed evidence for prehistoric (i.e., c. 820–520 cal. bc) beaver activity in the form of gnawed timber stratified in the basal sediments. The lowest horizons of the peat deposits contained the intact root systems of a dense hazel wood, indicating that the mire probably owed its origins to the rapid flooding of a relatively dry landscape. The exact cause of this inundation remains uncertain, although beaver damming is a strong candidate. The possibility that beaver activity is implicated in the flooding of the site is potentially of great significance both palaeoecologically and archaeologically. The site may represent the first recorded example in British prehistory of a non-human animal playing a role in inducing mire development. It also adds weight to the possibility that some small-scale woodland clearances identified in British pollen diagrams (which have been attributed to anthropogenic activity) may be due to the effects of beaver.