Transmission electron microscopy has been used to identify poly-3-hydroxybutyrate (PHB) granules in cyanobacteria for over 40 years. Electron-transparent (sometimes containing a slightly electron-dense area in the inclusions) or slightly electron-dense spherical inclusions found in transmission electron micrographs of cyanobacteria are often assumed to be PHB granules. The aim of this study was to test this assumption in Synechocystis sp. PCC 6803, and to determine whether all inclusions of this kind are indeed PHB granules. Based on the results gathered, it is concluded that not all of the slightly electron-dense spherical inclusions are PHB granules in Synechocystis sp. PCC 6803. This result is potentially applicable to other cyanobacteria.