Abstract: Aprotodon, a group of mid—Tertiary rhinoceroses with particularly wide symphyses, was previously known only from Pakistan and Kazakhstan. Its affinity has been little understood because of the paucity of the fossils available for study. The senior author of the present paper first mentioned its occurrence in the Lanzhou basin in 1990. In fact, its first finding was made early in 1986. In that year the junior author of the present paper collected some isolated teeth of this genus from Zhangjiaping, about 10km north of Lanzhou. Later in 1988 a well—preserved lower jaw with characteristic Aprotodon huge tusks was unearthed from a layer of thick white sandstone near the village Duitinggou, 10krn east of niangjiaping. Various specimens, including skull fragments and upper teeth attributable to Aprotodon, have been found in the Lanzhou basin since then. Now Aprotodon became the best represented genus among the large fossil mammals of the Lanzhou basin, and our knowledge of this genus is considerably augmented owing to these findings. The material to be studied below is important not only for the age determination of the Cenozoic deposits in the Lanzhou basin, but also for a better understanding of the genus Aprotodon.