One of Canada's rarest lichens, Heterodermia leucomela (L.) Poelt, was first discovered in Belleville, Ontario by George Lawson in 1862. Thought to be extirpated in Ontario, it is known from fewer than a dozen records in Canada, with only a few discoveries in northeastern North America in the past 100 years. My discovery in 2018 is of historical significance and provides hope that microhabitats or refugia required for presumed extirpated species may still exist in highly disturbed landscapes in southern Ontario.