An apparatus for growing crystals from hydrothermal solutions comprises a vertically extending autoclave having a crystallization zone in its top portion and a dissolution zone in its bottom portion. The autoclave is surrounded by a heat insulation layer, is provided with a system of heating elements and is enclosed in a thermostatically controlled chamber which is divided into vertically extending sections. Each section is provided with a heating and thermostatic control system so as to ensure the maintenance of constant and different temperature values in each section required to create and maintain the desired temperature fields in the crystallization and dissolution zones of the autoclave.