Several aspects of the ultrastructure of mycobacterial cells as seen by transmission electron microscopy of ultrathin sections that are relevant in the characterization of normal vs. altered bacteria are discussed. The importance of the use of adequate fixation conditions is stressed and illustrated with examples showing that normal, but inadequately fixed, mycobacterial cells may exhibit micromorphological alterations similar to those typical of cells affected in several situations such as autolysis, heterolysis and antibacterial treatments.