Abstract: Coreius heterodon and C. guichenoti are widely distributed in the Changjiang River basin and have long been the most valuable commercial fishes in the upper reaches of the river. They inhabit running waters and produce drifting eggs. The spawning grounds are mainly distributed in the upper reaches of the Changjiang River. Some of the eggs and fry are drifted down to the middle and lower reaches of the river, where they develop and grow. When they grow to a certain size, they ascend to the upper reach to spawn. Since the construction of the Gezhouba Dam, their migration route has been interrupted. In order to assess the effect of damming on the breeding and stock resources of Coreius heterodon and C. guichenoti in the upper reaches of the Changjiang River, a survey was made from 1981 to 1986. The results are summarized as follows: 1. There are 25 spawning grounds of Coreius heterodon and C. guichenoti within the course of 1432 km in the upper reaches of the mainstream of the Changjiang River. Among them nine are in the section above Yibing, sixteen in the section between Yibing and Yichang. The distribution of the spawning grounds of Coreius guichenoti is limited to the section above Chongqing, and that for C. hterodon in the section above Yichang. 2. According to the data in 1984, the number of eggs and fry of these two fishes in the mainstream of the Changjiang River was estimated to be 4.055 billion. 3. In the mainstream of the Changjiang River, the spawning of Coreius heterodon and C. guichenoti take place from April to early July. Due to the difference in water temperature, the spawning period is different in different sections. At Bingshan, spawning may occur in early April and is basically terminated by late May. At Yichang, spawning does until late April and is terminated by middle Jume. The fishes begin to spawn when the water temperature reaches 17℃, whereas temperatures of 19—22℃ are the most favorable spawning ends, when the water temperature reaches 25℃. In the breeding season, a suitable change in the water current velocity will induce the ripe fish to spawn simultaneously. The spawning activity occurs not only when the water level rises, i.e., the the current velocity increases, but also when the water level falls. This indicates that hydrological conditions needed for the spawning by these two fishes are not very strict. 4. After damming at Gezhouba, the original Yichang and Taipingxi spawning grounds near the dam have merged into the Liantuo spawning ground. In the first two years following the impounding of the reservoir, the centre of this spawning ground moved upstream and the number of eggs laid by these two fishes decreased in the reservoir area near the dam. The fry of Coreius heterodon were blocked by the dam and they grew and matured in the reservoir area. As a result, there has been a notable increase in the size of the spawning population. In 1985, the centre of the Liantuo spawning ground above the dam moved towards the dam again. This trend was particularly apparent in 1986. The egg production also increased. Coreius heterodon blocked by the dam can grow and develop normally at Yichang below the dam, where a new spawning ground has been formed. From the analysis of the survey results, the authors believe that the main impacts of the damming on Coreius heterodon and C. guichenoti are as follows. The dam prevents the migrations of the recruiting population below the dam and the fry above the dam; the distribution of the spawning grounds in areas near the dam; and the spawning scale have changed; and the sluice gates of the dam may injure some fish eggs and fry. Measures were proposed for the protection of the spawning grounds of these two fishes in this river.