Abstract An ultrasound pulse-echo method was integrated into a radiation-heated float zone process for the growth of silicon single crystals. Guided waves of f⩽100 kHz have been used as ultrasound signal, the energy of the pulsed signal did not exceed 1 mW/cm 2 and does therefore not influence the growth process itself. The relative position of the solid–liquid interface could be detected with a spatial resolution of ±5 μm for a sampling rate of 0.1 Hz and ±35 μm for 1 Hz, respectively. The interface curvature does not influence the signal quality, and the echo represents the integrated reflection from the entire growth front. The growth process and the movement of the solid–liquid interface can be observed in situ, and changes of the pulling velocity are directly reflected in the time-of-flight of the ultrasound echo. For switching the pulling drive on or off, the corresponding transients have been determined. For the described growth process, between 100 and 250 s are required to reach steady-state conditions. The method is sensitive enough to detect variations of the interface position related to changes of the heater power in the range of ±1% of the total power.