Bactericides were applied to experimental open recirculating cooling-water systems at concentrations found to be effective under laboratory pure-culture conditions. Total aerobic plate counts and bacterial population structures were determined over a period of 48h. In all cases the total aerobic count increased one day after the bactericide addition, and decreased rapidly after ca. 36 to 40h. Population shifts occurred during the course of all four treatments. In all cases different species became dominant concurring with fluctuations in the planktonic plate count, indicating the stress reaction of the biofilm. The species diversity decreased after treatment with dichlorophen, triocarbamate and methylenebis-thiocyanate, and increased upon treatment with humic acid. Species susceptible to bactericides in pure culture were found to be the dominant planktonic survivors. An example is Pseudomonas stutzeri which was the dominant survivor after treatment with thiocarbamate and with diclhlorophen.