in certain cyanobacteria responsible for nitrogen fixation in those species.

Heterocysts are seen particularly in cyanobacterial forms. They represent an unusual example of cellular differentiation, that is, where cellular differentiation otherwise is relatively rarely seen among prokaryotes.

Two additionally unusual features of heterocysts are that they form only as needed, that is, in response to sufficient (i.e., nitrogen ) and that to allow nitrogen fixation they must establish an anaerobic, environment (since the required enzyme, , is by molecular oxygen).

Since photosynthesis in cyanobacteria generates oxygen (i.e., O₂), the heterocysts do not photosynthesize. This means that they must be supported in terms of their and energy needs by neighboring cells. Movement of energy-rich organic compounds (presumably sucrose) occurs via channels found between the heterocysts and those neighboring cells. Channels similarly serve to transport fixed nitrogen out of heterocysts and into neighboring cells.

Cyanobacteria that can produce heterocysts are found in orders and . These include the familiar cyanobacterial genera, and .