The aim of the work is to analyse changes in the size of the population of the Aquatic Warbler Acrocephalus paludicola in the context of extensive agricultural use of Bagno Bubnów (Bubnów Marsh) and Bagno Staw (Staw Marsh) in PoleskiNational Park. Between the years 2007 and 2009 the population of the Aquatic Warbler was small and amounted to only 172–182 singing males (♂♂). Since 2011 there has been a growth in the size othe population, which has reached 351–389 ♂♂. The surface area of extensive use of the Aquatic Warbler’s habitat was variable and comprised between 11.3 ha in 2002 and 537.7 ha in 2 014. The Aquatic Warbler population in Poleski National Park is one of the most important worldwide, and one of the key ones in Poland and the EU; therefore, it is essential to maintain its extensive use at the level of at least 300–500 ha. On the other hand, the population of over 200 ♂♂, i.e. about 2% of the global population, should be considered as ‘minimally safe’.