Potato tubers cv. Maris Piper were inoculated with Erwinia carotovora subsp. atroseptica (E.c. atroseptica) and incubated anaerobically at 20 °C for 72 h to initiate rotting, then aerobically at 10 °C for 72 h to permit the resistance response of the tuber to develop. Examination by transmission electron microscopy of osmium-treated sections of rotted tissue revealed many apparently healthy bacteria with evenly dispersed cytoplasm, an intact outer membrane and an outer capsular layer which did not label with cationized ferritin. Similar numbers of bacteria showed a different appearance with a more amorphous cytoplasm which was less electron dense in the centre than in the outer region; the outer capsular layer was frequently discontinuous or missing, and cells labelled heavily with cationized ferritin. Healthy bacteria which stained for carbohydrate with Pa-T-Ag contained many polyglucoside granules, whereas the abnormal bacteria had either fewer or less-dense granules. The appearance of the abnormal bacteria observed in the rots was similar to that of E.c. alroseptica after incubation with rishitin in vitro.