At the beginning of July in 1970, reddish brown flecks appeared on the leaves of rice plants which were planted in the paddy fields in Kisarazu City and its environs, Chiba Prefecture. Since then, the outbreak of singular disease, resembling "Akagare" disease, has been observed in extensive paddy fields in Chiba Prefecture in every early summer (fig.1 and 2). The following results were obtained from the field surveys which were carried out to make clear the cause of the disease. The most severe injury appeared always on the second fully-expanded leaf from the top. As to the extent of injury, there were differences among varieties, but was no obvious difference according to cultivating or edaphic factors. Although the disease occurred simultaneously in extensive areas, any pathogenic mold fungi or bacteria could not be detected in the injured leaves. Immediately before the outbreak of the disease, in those areas air pollution chiefly photochemical oxidants occurred. At the same time with the outbreak of the disease, the leaves of several kinds of crop plants such as spinach, radish and corn suffered from the photochemical oxidants. An inference that the outbreak of this singular disease was caused by air pollution due to the photochemical oxidants was drawn from the results of field survey. Basing on this inference, rice plant were exposed artificially to ozone which is known as the principal element of photochemical oxidants. The results are as follows. Rice plant was so sensitive to ozone that reddish brown flecks appeared on their matured leaves when exposed to ozone of 0.15 ppm for a few hours. Injury by ozone (fig.4) was identical in symptoms and histological changes with the reddish brown flecks which occurred in the field (fig.3). When rice plant was grown in the filtered air chamber which is able to remove photochemical oxidants, the injury was coinpletely suppressed throughout the summer. From these results, it was concluded that the reddish brown flecks had been caused by photochemical oxidants