Electron microscopy has largely contributed to the study of bacterial anatomy. However, as varied alterations can occur during cell preparation, at the level of cell structure and at the molecular level, it is difficult to know to what extent electron micrographs correspond to the true appearance of the living state. The recent development of cryomethods which avoid some of the alterations which may occur during conventional fixation and embedding procedures, has shed new light on bacterial anatomy. These have definitively proved that mesosomes do not exist, but are artefactual structures induced by the fixative. New features of the bacterial "nucleus" relating to its shape and fine structure appeared in thin sections of Gram-positive and Gram-negative bacteria prepared by cryosubstitution. New information has also been obtained on the cell wall structure of different bacterial species.