Pseudomonas syringae pv. atropurpurea, a causal agent of halo blight of Italian ryegrass, produces coronatine (Cor) which is a toxin inducing halo blight lesions on the leaves. The relationship between Cor productivity and a specified plasmid DNA in this bactreium was studied. The plasmid of approximately 58 Mdal named as pCORl was detected in all of the 34 Cor⁺ wild strains tested including NIAS 1309. On the contrary, none of pCORl was detected in all of the Cor^- strains obtained as spontaneous mutants from NIAS 1309 or isolated from the host plants. Furthermore, all of the 38 Cor^- mutants obtained from NIAS 1309 by the treatment with acridine orange (75 to 200ppm) simultaneously lost pCORl plasmid completely. Among them, however, 29 mutants still harboured a smaller plasmid (about 40 Mdal) seemed to be a defective pCORl. These results strongly suggest that the plasmid pCORl codes Cor productivity of the bacterium.