Abstract: The article reviews the differentiation of hormogonia of several filamentous heterocystous cyanobacteria and their regulatory mechani sms. The hormogonia are distinguishable from vegetative trichomes by the cell shape, the presence of gas vesicles and cell motility. Numerous environmental factors, including light and nutrients, can stimulate or inhibit hormogonium differentiation. Some strains belonging to genera Nostoc, Tolypothrix and Calothrix display a complex developmental cell cycle that includes both the differentiation of gasvacuolated hormogonia, heterocyst differentiation and complementary chromatic adaptation. These three types of adaption depend on two different photoreceptor system s. The control of both hormogonium and heterocyst differentiation depends on the photosynthetic electron transport chain, but complimentary chromatic adaptation appears to be controlled by a photoreversible pigment. The cyanobacterial PII protein that is involved in the control of both synthesis and activity of glutamine synthetase plays a role in the coordination of hormogonium and heterocyst differentiation and complementary chromatic adaptation. The differentaition of hormogonia from cyanobacteria and their regulation mechanisms are subjects that need more thorough investigation because of the incompleteness of the knowledge of hormogonia.