Phytosociological and habitat studies of <em>Sphagnum denticulatum</em> and <em>Warnstorfia exannulata </em>phytocoenoses from Polish <em>Lobelia</em> lakes have been conducted. The present results were compared with data on <em>Isoetes lacustris </em>community. It is demonstrated that both communities of mosses are distinct with respect to their floristic composition. Moreover, they differ significantly from the patches of <em>I. lacustris </em>with regard to many properties of water and substrates. The phytocoenoses of <em>S. denticulatum </em>are associated with softer waters, poorer in Ca<sup>2+</sup>, Na<sup>+</sup>, NH<sub>4</sub><sup>+</sup> and dissolved organic matter, but richer in total Fe than those of <em>I. lacustris</em>. They are also distinguished by lower values of electrolytic conductivity. The substrates they inhabit are more strongly hydrated and richer in organic matter, total N, dissolved SiO<sub>2</sub>, Ca<sup>2+</sup> and Na<sup>+</sup>, but poorer in PO<sub>4</sub><sup>3-</sup>. The <em>W. exannulata</em> phytocoenoses are also confined to softer waters with lower Ca<sup>2+</sup>, NH<sub>4</sub><sup>+</sup> and dissolved organic matter contents than those of <em>I. lacustris</em>. Their substrates are more strongly hydrated and have higher contents of organic matter, total N and Na<sup>+</sup> but lower amounts of PO<sub>4</sub><sup>3-</sup>. In addition the waters of <em>W. exannulata </em>phytocoenoses are characterized by higher pH values and lower concentration of dissolved SiO<sub>2</sub>. The substrates are less acidic and richer in NO<sub>3</sub><sup>-</sup>, but poorer in total Fe. The differences between the habitats of the <em>S. denticulatum </em>and <em>W. exannulata</em> phytocoenoses are also distinct. The former inhabit more acidic waters, richer in dissolved SiO<sub>2</sub> and total Fe, but with lower values of electrolytic conductivity and concentration of Na<sup>+</sup>. Their substrates are distinguished by higher contents of organic matter and Ca<sup>2+</sup>, as well as by lower amount of NO<sub>3</sub><sup>-</sup>. In view of the fact that the above phytocoenoses of mosses are distinct with respect to their floristic composition and are associated with specific habitats in which they form dense mats close to the bottom of the lake, they should be classified as distinct associations (Sphagnetum denticulati and Warnstorfietum exannulatae) in the system of aquatic communities (Class: Utricularietea intermedio-minoris).