The structure and development of the akinete of a blue-green algae, Cylindrospermum sp., was investigated with the light and electron microscopes. Only the sub-terminal cell which appears to divide immediately before the onset of development becomes an akinete. There is first an elongation and then a rounding out of the sub-terminal cell. The akinete that develops is larger than a vegetative cell, but it has the same cell wall components and the same kinds of inclusions. There are an unusually large number of structured granules and ribosomes present. The akinete, in contrast to the vegetative cell, has a heavy fibrous coat outside the inner investment, and an electron dense substance accumulates within the fibrous coat. The fibrous coat is of medium electron density. The electron dense substance is drawn outward into rays, and the fibrous coat and the inner investment are separated by an electron transparent layer in the mature akinete.