In 2001, a new bacterial disease was observed on hydroponically cultured pak-choi [Brassica campestris L. (chinensis group)] in Fukushima Prefecture, Japan. Symptoms were expressed as water-soaked lesions that extended from the midrib to the tip of leaves. A causal bacterium was isolated not only from the infected tissues but also from the hydroponic solution. Stab inocula-tion of the midrib reproduced the original symptom. The symptom was also expressed after artificial inoculation of the roots, suggesting possible root infection from the bacterium in the hydroponic solution. The pathogen was identified as Xanthomonas campestris pv. campestris (Pammel 1895) Dowson 1939.