In the method and apparatus for measuring the position of the phase interface during growth of a crystal from a melt in a crystal growth container according to the VGF method an incident optical signal is propagated to the phase interface between the melt and the crystal through a window (16) in the container (10) and a received optical signal reflected from the phase interface (14) is measured to determine the position of the phase interface. The position of the phase interface is established from the reflected signal by triangulation with a confocal optic system, by interferometric balancing or by transit time of the optical signal. The window (16) is preferably mounted in a preferably tilted orientation at the end of a tube (15), which is immersed in the melt (12).