This paper presents the results of 11 years study of aquatic warblers in the Roskosz Nature Reserve in eastern Poland. The study was carried out in fen mires near Chełm, in the proximity of the Polish-Ukrainian boundary. The number of aquatic warblers was correlated with the mean water level and with the mean groundwater level. We found that the higher the surface water level, the smaller the number of singing males. The largest number of males (density 10.9 singing males per 10 ha) was found when the mean water level was 23.7 cm, and the smallest number (density 2.1 singing males per 10 ha) when the mean water level was 35.7 cm. These results can be useful in studies that focus on the optimal habitat conditions for breeding of aquatic warblers. This is the first systematic study in calcareous Cladium mariscus fen mires.