The effects of oxadiazon; 5-t-butyl-3-(2, 4-dichloro-5-isopropoxyphenyl) 1, 3, 4-oxadiazoline-2-one, which represents the herbicidal action only under the light condition, on the ultrastructure of chloroplasts of barnyardgrass (Echinochloa crus-galli) was studied electronmicroscopically. When the etiolated seedlings, grown for three days in oxadiazon solution under dark condition, were exposed to light, greening and elongation of the leaves were inhibited markedly. Most of leaf tissues were considerably distorted by the action of oxadiazon. Especially disintegration of vascular bundle was remarkable. In chloroplasts of untreated leaf cells, grana was formed and starch grains were also found in some case. However, in the case of herbicide treated plants, chloroplast development was suppresscd. These chloroplast were characterized by having prolamellar bodies and circular lamella structure. However, even in cells whose nucleus and cytoplasm were distinctly affected, rupture of the chloroplast with disintegration of envelop membrane was not observed and their shape were kept their original form. It is proposed from the above mentioned results that oxadiazon influenccs in not only chloroplasts but also nucleus and cytoplasm in spite of the well known action of the substance which closely related with the light exposure.