The valley mire in northeastern Poland. The ecosystems of the peat area of the Biebrza valley in the north-eastern parts of Poland are discussed. The distinct zonation in vegetation from the river towards the morene includes 1) the eutrophic marsh area flooded in winter, 2) the irregularly flooded zone with tall sedges, 3) the mesotrophic, species rich, zone with smaller sedges, where groundwater discharges, 4) Alder woodlands bordering the sandy areas. The flora and fauna of these zones is mentioned with its characteristic ecology and functioning, and compared with mire systems in The Netherlands.