Abstract:Cellular ultrastructure of Nostocflageliforme Born. et Flab. under different growth conditions have been observed in order to study the characteristics of growth and development and the mechanism of gelatinous sheath formation. All kinds of cells were in an active metabolic state of growth with regular cell division in distilled water after two hours. The formation of gelatinous sheath was associated with the cell wall of the vegetative cells and a special type of macrocells, both of which were physiologically active at a culture-medium pH of 7.0. In an acid pH environment, however, the vegetative cell wall became inactive, no macrocells appeared but instead, a large number of a special type of microcells were observed.