The fundamental of the paper is a mineralogical characteristic of the pegmatite and quartz veins in the basin of the rivers Kara-su and Ak-su (Hanaka-su) which flow along the northern slope of the Hissar mountain range, in Central Asia. To the paper is annexed a geological map, on the scale of 1:100000, indicating the pegmatite veins described therein. At the beginning of the paper is given a concise outline of the structure of the region. The pegmatite veins explored are situated at different distances from the granitic batholith, and therefore the author considered it necessary to analyze the whole course of the formation of pegmatite minerals in the given region, beginning with high-temperature processes and finishing with those of low temperature. The author proved that in most veins described a posterior hydrothermal phase had overlaid the already formed pegmatitic bodies and that a change of the mineralogical composition of the pegmatite veins had been a consequence of that. Bearing in mind the peculiarities of the mineral-forming processes of the given region the author analyzes in detail all processes of replacement of primary minerals in pegmatites. A number of microphotographs and drawings in the text illustrates these „replacement-textures“ of primary minerals. From an analysis of the peculiarities of the mineralforming process in the pegmatites of the region in question the author comes to the conclusion that the cassiterite content in the veins increases with depth. At the end of the paper the author, taking into consideration the materials studied, gives a general scheme of the development of the pegmatite process which has produced the pegmatite bodies in the region described (chapter entitled „Genesis of pegmatites“). The fundamental idea of this scheme might be successfully extended to the interpretation of the course of pegmatite processes in other regions.