Bacterial communities in water-cooling systems treated with bactericides often become resistant to these bactericides. This has been ascribed to selection for resistant cells. Certain bacteria, having a high inherent susceptibility to water treatment bactericides, become dominant in systems after bactericide treatment. We investigated the idea that bacterial isolates adapt to growth in the presence of bactericides. Pure cultures of Pseudomonas stutzeri and Bacillus cereus were cultured repeatedly in the presence of sub-inhibitory concentrations of 2,2’-methylenebis(4-chlorophenol), sodium dimethyldithiocarbamate and isothiazolone. Both isolates adapted to growth in the presence of increasing concentrations of the bactericides. P. stutzeri adapted from 22 µg/ml 2,2’-methylenebis(4-chlorophenol) to 80 µ g/ml, from 12 µg/ml Na dimethyldithiocarbamate to 310 µg/ml, and from 50 µl/l isothiazolone to 250 µl/l. B. cereus adapted from 20 µg/ml 2,2’-methylenebis(4-chlorophenol) to 75 µg/ml, from 6 µg/ml Na dimethyldithiocarbamate to 132 µg/ml, and from 50 µl/l isothiazolone to 300 µl/l. The phenomenon of resistance to water treatment bactericides can be ascribed not only to selection but also to adaptation.