The occurrence of a new bacterial disease, halo blight, has recently been recorded on rice plants (Oryza sativa L.) in several districts of Aomori Pref., Japan. The disease is characterized by small brown lesions surrounded by a large and yellow halo on the leaf blades. A comparative study of the causal agent indicated that as a whole the bacteriological characteristics of the present bacterium is identical with those of Pseudomonas syringae pv. atropurpurea and P.s. pv. coronafaciens which also produce halo blight on various gramineous plants. In addition to rice plants, the present bacterium was also pathogenic to Hordeum vulgare, Avena sativa and Phaseolus vulgaris, inducing similar symptoms. However, P.s. pv. atropurpurea and P.s. pv. coronafaciens were not pathogenic to rice plants. From these characteristics it was concluded that the present bacterium is a new pathovar of P. syringae, and the name P. syringae pv. oryzae n. pv. was thus proposed.