This paper deals with a bacterial blight of chestnut and its causal organism. The disease is mostly conspicuous in buds and young shoots though also occurs on the leaves, veins, petioles and bracteal leaves, the last being usually attacked at first. The sign of the disease in the early stage is water-soaked spots on the leaves and young shoots in which the cortical parenchyma is destroyed, forming the bacterial cavities and resulting in brown cracks. Leaves attacked when young become distorted and leaves of infected buds shrivel and die. White and yellow bacteria are isolated from the diseased lesions, of which the former is proved to be pathogenic to chestnut and found associated with the latter, usually at a later stage of the disease. The morphological, cultural and physiological characters of the causal organism has been studied and Bacterium Castaneae n. sp. is proposed for the organism. The technical description of Bacterium Castaneae n. sp. is as follows: A short rod, 1.0-1.8μ long by 0.8-1.2μ wide, motile by one to five polar flagella; singly or in pairs; no spores; no capsules; Gram-negative; facultative anaerobic; beef agar colonies white, round, slightly undulate and viscid; bouillon clouds without pellicle; potato decoction agar colonies white, radiately rugose; gelatin not liquefied; diastatic action absent; milk peptonized without coagulation; indol and hydrogen sulphide not produced; nitrate and methylene blue reduced; acid from dextrose, saccharose and glycerine without gas, neither acid nor gas produced from lactose, no gas from maltose and potassium nitrate; optimum temperature 25°-27°C., minimum below 3°C., maximum 35°C., thermal death point 50°-51°C.; parasitic on chestnut causing leaf sports and brown cracked lesions on the veins, petioles and shoots and destroying the buds; type locality Kasuya, Fukuoka, Japan.