Two distinct round patches were observed in a potato field where the haulm showed symptoms like those of blackleg. The remainder of the field was in vigorous growth. Affected stems had long cracks and Erwinia carotovora var. atroseptica was isolated from some. This had probably developed subsequently on damaged tissue and no rotting was apparent below soil level. Most of the tubers were decayed but no Erwinia and only green pseudomonads were detected. Local meteorological records showed that thunderstorms occurred on only one day (1 August) in the six weeks before observation of the field symptoms. It was concluded from all evidence that the condition described was caused by a lightning strike on that day.