In the current study, it was aimed to determine the efficacy of the different dosages of sodium hypochlorite, benzisothiazol/isothiazolin-ones and chloramine T trihydrate compounds against 12 months old mature biofilm. Biofilm samples which grown in model recirculating system were exposed to biocides for 1 and 24 hours. At the end of the contact time, the numbers of heterotrophic bacteria in samples exposed to biocides were compared with the control samples.It has been determined that all concentrations of tested compounds provided >1 log reductions in heterotrophic cultivable bacteria at 1 hour contact time. After 24 hour contact time 20 and 50 mg/L sodium hypochlorite provided >3 log; 15, 100 and 150 mg/L benzisothiazol/isothiazolin-ones compound provided >2 log; 1000, 2000, 3000 mg/L chloramine T trihydrate compound provided >3 log reductions.There was no statistically significant difference in efficacy between doses. Different dosages of the same biocide provided similar log reductions in heterotrophic cultivable bacteria. Study results demonstrated that similar antibacterial effect can be achieved with applying lower dosages of tested compounds. Thus, by determining the minimum amount of biocide which is sufficient for biofouling control, equipment and environment can be protected by preventing excessive biocide usage; additionally the development of drug-resistant microorganisms would be prevented.