Abstract: A method and apparatus for growing single crystal of GaAs, etc uses the Czochralski techniques. Control of crystal (1) diameter is by a closed loop (9, 10, 24, 7, 6) control of melt (4) temperature in response to crystal weight signals (9) W or dW/dt. The invention injects a test signal St (22, 26) into the control loop and performs a signal processing (21), e.g. cross correlation, on St and crystal weight signal. Peak amplitude of correlation values is related to the growing crystal shape. This is used by comparison with reference values (24) to control the growing-out phase from seed diameter (16) to full diameter of the crystal (1).