This paper presents results of investigation on peat and lacustrine sediments from the Kładkowe Bagno peat-bog located in the Puszcza Knyszyńska Forest. Using analysis of plant remains from sediment samples, vegetative and generative finds were identified which allowed describing peat units. Basing on these results, reconstruction of subfossil vegetation and palaeoenvironmental changes in the mire was made. Altogether 4 subassociations of Sphagnetum magellanici were described, which delivered information about humidity of the mire surface during peat forming processes. Stages of deposit development were dated by radiocarbon method. Accumulation of the oldest sediments in the southern basin took place in the Late Glacial. Peat of the northern basin started to accumulate in the Atlantic period. The both parts of the mire aggregated probably 400 years ago.