Study of the pegmatite deposits of tinstone from East Transbaikal yielded a number of results indicating a relationship between the form and genesis of this mineral. A series of cassiterite crystals found in pegmatite veins confirms the association of pyramidal crystals with high-temperature formation conditions. The described crystals of tinstone from the Zavitinskoye deposit, of high-temperature character, were of interest in that they exhibited a prismatic habit, which is characteristic of low-temperature pegmatites. Measurement of a single crystal showed that it is a dipyramid with angles between the normals to the dipyramid faces equal to 58°16'.