Aims: The aim of the present study was to investigate the behaviour of two pathogenic vibrios (Vibrio cholerae O1 and Vibrio parahaemolyticus) during deputation and to compare it with that of Escherichia coli, used as an indicator of suitability for consumption. Methods and Results: Samples of Mytilus galloprovincialis were experimentally contaminated with E. coli, V. cholerae O1 and K parahaemolyticus, depurated in a pilot plant using ozone and analysed at selected intervals. Numbers of E. coli and vibrios were estimated using an MPN method. The presence of vibrios was confirmed by the use of PCR. The target genes used were ctx for V. cholerae O1 and the restriction fragment pR72H for V. parahaemolyticus. There was a substantially smaller reduction in the numbers of both vibrios (approximately 1 log) during the deputation process than of E. coli (approximately 3 log). Conclusions: The results confirm the inadequacy of E. coli as an indicator that molluscs have been cleansed of other microbiological agents. Significance and Impact of the Study: The study confirms the risk associated with the consumption of mussels and the need to correctly conserve and cook them prior to consumption.