Erwinia amylovora, the etiological agent of fireblight disease, was found to produce under aerobic conditions in axenic culture 2-amino-1,7-dihydro-6H-purine-6-thion (6-thioguanine, 6-TG), easily recognized by a UV absorbance at 340 nm (pH≤7) and readily distinguished from NADH by both its acid stability and the pH dependence of its UV spectrum. 6-TG is known for a variety of physiological activities, but in this study no toxicity toward Bartlett pear suspension culture was detected. Although a synergistic role of 6-TG in the development of fireblight is not excluded, it may not be responsible for the black necrosis typical for the disease. This conclusion was corroborated through the study of severalE. amylovora mutants with respect to their capability to produce 6-TG.