The heterocysts of Anabaena cylindrica were freed from filaments by differential disruption of vegetative cells using four techniques: mechanical disruption by French press, sonication, osmotic shock and lysozyme. The ultrastructure of isolated heterocysts was compared with that of heterocysts in intact filaments. The first three methods produced heterocysts whose internal structure showed different degrees of damage, involving in particular disruption of the heterocyst cell wall and plasmalemma. Isolation by the lysozyme method yielded heterocysts which appeared in the electron microscope to be intact and comparable with those of the untreated controls. These results suggest that earlier reports on the physiological properties of heterocysts isolated by means of the French press or sonication may require re-examination.