Pollen-stratigraphic investigations are described from a site on the northern slopes of the Brecon Beacons, South Wales, in which over 5.0 m of Devensian Late-glacial and early Flandrian sediments have accumulated. Relative (percentage) pollen concentration, and deteriorated pollen diagrams were constructed and these data were augmented by radiocarbon dates from the major stratigraphic boundaries. The evidence shows that relatively mild conditions characterized the Late-glacial Interstadial, but these were succeeded at c. 10600 B.P. by the markedly colder phase of the Loch Lomond Stadial during which a severe periglacial climatic regime prevailed in the area. Climatic amelioration at c. 10000 B.P. led to a rapid sequence of vegetational changes and the establishment of a birch–hazel woodland on the lower slopes of the Brecon Beacons, possibly in the space of 1000 years. Of particular interest in the profile are fluctuations in both pollen percentages and pollen concentration values during the Late-glacial Interstadial which are interpreted as reflecting major landscape changes around the site. Possible explanations for the fluctuations, which may well have been climatically-induced, are discussed in the context of other Late-glacial records from Wales, Highland Britain and northwest Europe.