An Ascochyta fungus was obtained during a survey of leaf spot diseases of Fagus crenata in the mountains of Aomori prefecture in 1995. The pathogenicity of the fungus to F. crenata was confirmed by inoculation. Its teleomorph was first found on artificially infected leaves after the leaves were placed in an incubator at 5°C with a 12-h photoperiod (approximately 500 Ix, daylight strip lamps) for 4 mo. The fungus was found to be the causal agent of yellow leaf spots of F. crenata and Quercus mongolica var, grosseserrata in the field. Comparison with similar fungi so far described from Fagaceae indicated that the anamorph is Ascochyta fagi and the teleomorph is an undescribed species of Didymella. The name Didymella fagi is introduced for the teleomorph. Correlation between the two morphs has been proved by cultural, morphological and pathological studies.