Abstract –Transferring actively growing bacteria or Streptococcus mutans ATCC 27351 into a xylitol‐containing reaction mixture caused distinct alterations in bacterial ultrastructure without notable effect on the total viability of the strain. Incubations in media containing 50 mg/ml of glucose, fructose, sucrose, lactose, sorbitol or mannitol as the primary carbon source did not affect bacterial ultrastructure. These fermentations were reflected biochemically in the amounts of insoluble glucans, as expected. A negative correlation was found between the cell mass and the lipoteichoic acid formation. But these aspects could not be visualized in the electron microscope. In the xylitol series, however, degrading cells and autolysis, intracellular vacuoles and lamellated formations in the cytoplasmic membrane were frequently seen independent of the concentration of xylitol in the reaction mixtures. In freeze‐fracturing replicas, however, the membrane intercalated particles of the cytoplasmic membranes seemed to be unaffected and like those in the controls. Minor ultrastructural changes in the fracture‐faces were detected. Despite the alterations in ultrastructure of the xylitol‐incubated bacteria, there was no difference in their viability when compared to the