Chara tomentosa is a species placed on the red list of endangered algae in Poland. However, its ecology is still barely known. The aim of the study was to advance this knowledge through determination of the variability of Chara tomentosa specimen between the stands of varied habitat properties. The structure of the specimen was tested in six lake populations of Chara tomentosa in Pojezierze Mazurskie (Mazurskie Lake District). A total length and biomass of the specimen as well as its habit and reproductive potential were determined in each of them. In the habitats rich in calcium, nitrogen and phosphorus, long (52–74 cm), of high biomass (0.6–1.2 g), extensively branched (2 branches on average) specimens of high reproductive potential (1.5–3.2 generative branches) occurred. Whereas, in the habitats poorer in the said elements small (29–42 cm), light (0.5 g) usually weakly branched specimens of low reproductive potential (0.5 generative branches) occurred.