The study of the spatial coherence properties of light scattered by ultrasound‐produced rolls in MBBA allows one to obtain information on the hydrodynamics of the process. In particular, the correlation lengthlnof refractive‐index fluctuations is first found to increase and then to decrease with the voltageVapplied to the piezoelectric transducer. The onset of ’’turbulent’’ motion is clearly visible at the voltage at whichdln/dVchanges sign.