Habituated sugarbeet callus examined by means of a light-microscope is characterized by absence of differentiated tracheary elements and of the reaction with syringaldazine. Habituated tissues also were found to exhibit low guaiacol- and syringaldazine-peroxidase activities, a deficiency of lignin as well as lower cellulose, dry mass and chlorophyll contents as compared to the normal auxin-requiring callus. These histological and cytological features led to consider this habituated callus as a vitrified tissue under stress. The question of a relationship between vitrification and habituation is posed.