AbstractThe thickness of the boundary layer surrounding polyhydroxyalkanoic acid (PHA) inclusion bodies in bacteria was determined from electron micrographs of ultrathin cell sections. The obtained values were compared with the thickness of boundary layers of other kinds of prokaryotic inclusion bodies known to be monolayers (sulfur globule, hydrocarbon‐containing inclusion, carboxysome, gas vesicle, chiorobium vesicle) and the thickness of typical bilayer membranes (cytoplasmic membrane, photosynthetic vesicle). The data are discussed in view of contradicting models published recently for the molecular architecture of the boundary layer of PHA inclusion bodie