AbstractThis paper contains the results obtained in the investigations of morphology, ultrastructure and vertical distribution of microorganisms in a stratified lake, Lesnaya Lamba, Karelia. The culture ofOchrobium tectum(PERF.) was produced for the first time, and the bacterial nature of this organism was shown by investigating its ultrathin structure. The presence of gaseous vacuoles was also revealed, which explains the migrational ability of ferritized cells. Several new microorganisms were detected, including the filamentous photosynthetic green bacteriumChloronema giganteum(nov. gen. nov. sp.), a consortium of a new symbiotic form of green bacteria namedChloroplana vacuolata, formerly unknown tiny purple bacteria and blue‐green algaeGloeocapsa, oxidizing iron. It was revealed that more than half of the bacterio‐plankton in the bottom layer possess gaseous vacuoles; they are represented by various morphological forms. The vertical distribution of iron bacteria —Siderocapsa (Arthrobacter), Ochrobium, Siderococcusand other species is regulated according to ecological niches by the concentration of oxygen and iron, and, in phototrophs, by the amount of light and the presence of a certain type of bacteriochlorophyll. All in all, the distribution of nine species of phototrophic bacteria in the lake is desc