Abstract
Light and electron microscopy study of Cyclotella ocellata Pantocsek (Bacillariophyta) species from Lake Khubsugul (Mongolia) revealed an appreciable morphological variability of qualitative (valve relief, presence of several morphotypes, structure and arrangement of rimoportulae, central and marginal fultoportulae, and alveoli) and quantitative features (diameter of vegetative and initial valves, number of central fultoportulae and rimoportulae) along with the occurrence of colonies. Probably, the lake is simultaneously inhabited by several subpopulations of C. ocellata with cells of various sizes and therefore the formation of auxospores occurring in several size ranges.