In order to take account of the likely increase in costs of biocides in the light of increasing legislation and concern for the environment, there is a need to maximise the efficiency of biocides for the control of biofouling. The use of ultrasound in conjunction with biocides offers such an opportunity. Tests have been carried out using ultrasound generated at 20 kHz in conjunction with the oxidising biocide ozone, in a laboratory pilot plant, to investigate the effects of mutuality. The preliminary results reported in this paper suggest that the combined effect of ultrasound and the biocide is better than either separately employed. Clearly substantially more work is required in order to maximise effectiveness for minimum cost. (C) 2003 Elsevier B.V. All rights reserved.