Abstract—A case of acute ulcerative gingivitis (AUG) with pericoronitis is presented showing how differential dark‐field microscopy can be used to aid in diagnosis and monitor therapy. The AUG site showed a unique flora with dark‐field microscopy, which in this case was shared by the pericoronitis lesion. Metronidazole therapy resulted in a marked change in the flora of both sites. Four days following therapy both sites showed a similar flora to a normal healthy site which remained unaltered throughout the trea