This work describes an ultrasonic method for measurement of membrane fouling in pressure-driven membrane separation system resulting from filtration of natural brown water. The ultrasonic signals acquired during the in-situ detection of membrane fouling were analysed using wavelet transforms (WTs). Wavelet analysis also was applied to differential ultrasonic signals to obtain additional information about fouling. The work was undertaken at the as part of the K5/1441 project funded by the Water Research Comission of South Africa (WRC).