The possible development of resistance of bacteria found dominant in industrial water systems to the oxidising biocides hypochlorous acid and 3-bromo-1-chloro-5,5-dimethylhydantoin was investigated. Pseudomonas aeruginosa, P. stutzeri and Bacillus cereus were cultured repeatedly in the presence of sub-inhibitory concentrations of hypochlorous acid and 3-bromo-1-chloro-5,5-dimethylhydantoin. The minimum inhibitory concentrations (MIC) of hypochlorous acid and of 3-bromo-1-chloro-5,5-dimethylhydantoin decreased following initial exposure, but varied greatly during the period of the investigation. The MIC's did stabilise towards the end of the study. Whereas the isolates used did not display classical resistance, they did respond variably to successive exposures, indicating that long-term treatment of water systems with hypochlorous acid or 3-bromo-1-chloro-5,5-dimethylhydantoin would yield variable degrees of control of microbial activity.