In the previous paper, the authors have reported that there were two geographical races in the rice stem maggot, Chlorops oryzae MATSUMURA. One of them distributes in the southern part of Japan and has three generations a year, whereas the other distributes in the northern part of Japan having only two generations. In the border region of the above two areas, populations consist of both races. Therefore, studies on the populations of such a intermingled region may contribute in increasing the knowledge of the geographical races in the rice stem maggot. From this viewpoint, the authors have conducted some ecological experiments on the maggot population of Kanose, the situation of which is in the intermingled region. The maggots from Kanose were transferred to Takada (threegeneration area) and were compared with those of Takada and Ômagari (two-generation area).<br>The results are as follows:<br>1. The emergence of the first generation populations of Kanose and Ômagari which were overwintered at Takada under natural condition occurred later as compared with that of Takada. Among these populations the emergence of Ômagari was the latest.<br>2. The authors have reported in the previous paper that the pupal duration of winter generation of Takada population was shorter than that of Ômagari. The pupal duration of Kanose population varied from short as Takada to long as Ômagari.<br>3. It was impossible to divide the Kanose population into two groups of two- and three-generation types with the emergence period of the first generation flies or with the variation of the pupal duration of winter generation.<br>4. The first generation larvae of Kanose were reared on young rice plants of susceptible variety. In this case, two kinds of larvae were observed; the one was quickly growing larvae which pupated soon after feeding only a few leaf blades remaining some arranged acars on the leaves injured at the later larval period, and the other was slowly growing larvae which did not remain arranged acars on the leaves and did not pupate before they fed young panicles. Besides these two kinds of larvae, somewhat slowly growing larvae which remained arranged acars and pupated before the heading of the host plant were seen.<br>5. Many of the quickly growing larvae seemed to come from the early emerged flies, while many of the larvae which came from the late emerged flies seemed to be the slowly growing larvae. The rate of slowly growing larvae in the first generation increased gradually with the delay of hatching period.<br>6. In respect of the quickly growing larvae, most of them pupated within thirty days regardless to hatching period.<br>7. In the next brood population which came from the cross of the Takada and Ômagari, most of the larvae became either quickly or slowly growing ones. This result was similar to the next brood population of the first generation flies of Kanose which had medium growing speed.<br>8. In F₃ population derived from the quickly growing individuals of F₁ which came from the cross of Takada and Ômagari, the quickly growing larvae appeared with high percentage. While, as to the F₂ population derived from the slowly growing individuals of F₁ the reverse result was obtained. The similar results were obtained as to the third brood population derived from the individuals of Kanose quickly growing in the first generation and the second brood derived from the individuals slowly growing in the first generation.<br>9. It may be concluded that the population of the rice stem maggot in so-called intermingled region represented by Kanose was the one derived from natural hybridization between the two races.