This investigation was conducted to know the distribution structure of rice stem borer larvae as well as the injury in the area which is divided into several parts that the administrative (such as district, aza or village, town, and city etc.) or geographical divisions were considered as the units of their ecological fields. In 1957 and 1958, the paddy field having a radius of 300 metres with a forecasting lighttrap in its center, at the Yata-district; in 1959, the lower plain (1500ha) along the Hidaka River, Hidaka Gun, Wakayama Prefecture; in 1960, the lower plain of the Kinokawa-River in Kii-district, Wakayama-City were respectively singled out for this investigation.<br>The numbers of the infested plants and stems and the larvae found in them were counted at the utmost-infested of the first brood (the middle of July), at the end period of the first brood (the middle of August) and after the larvae of the second brood were all scattered away (the middle of October), respectively.<br>The results obtained in this invetigation can be summarized as follows:<br>1. In the case of the utmost-infested period, the intensity of injury differs by location though it was not so remarkable as in the case of the second brood. Especially in the case of the larger area as much as 1500ha, the percentage of infested plant in one district was found to be six to nine times as much as the one of another. In the case of 1959, the percentages of infested plant in the plain districts were smaller than the sloped districts.<br>2. In the case of the end period of the first brood, the infested stem or larvae density was observed to be remarkably lowered by the environmental resistances. But also in this period high densities were equally found in the districts where an abundant population density was observed in the previous period, and in most of the districts, the population densities were not decreased equally by effects of parathion.<br>3. In the case of the second brood, the differences of densities between the districts were not always corresponded with the first. And the aggregated distribution was found peculiarly in district location. This tendency of aggregation in this period was far more remarkable than in the first brood period and the number of infested stems reached ten times as much as in the first brood period. And the locations where high injuries observed were constantly the same place all through this four years investigation.