Heterocysts of Chlorogloea fritschii were studied with the aid of light and electron microscopy. Two main types of heterocyst were recognized. One of these, termed here the H.1 cell, is the only type in filaments, but occurs also among endospore groups. The other, the H.3 cell, is restricted to endospore groups. The transition from one type of heterocyst to the other, therefore, does not coincide exactly with the transition in arrangement of vegetative cells from filaments to endospores. Both types of heterocyst differ considerably from other published accounts of heterocysts. The differences between H.1 and H.3 cells reside mainly in the arrangement, and relative development or disappearance of the various cell organelles. A summary is given of other types of cell seen with the electron microscope which share some features with typical heterocysts. Some of these would probably be recognized as heterocysts by light microscopy, whereas others would not.