Nostoc strains isolated from different cycad coralloid roots were grown in culture until sporulation to study the structure and ultrastructure of akinetes. These cells have cell walls enclosed by two different envelopes produced by the vegetative cells during akinete differentiation, and cytoplasm containing thylakoids, cyanophycin granules, polyhedral bodies, and polyglucoside and lipid bodies. Two main types of akinetes have been described differing in shape, size, envelope, and cytoplasmic features. They are considered as cells having a different grade of maturity and function. The factors which influence sporulation and the correlations between akinetes and heterocysts are discussed.