Aims: To study the effect of the isothiazolone biocide (Kathon WT) on the performance of laboratory-scale rotating biological contactors (RBCs) and their component biofilms. Methods and Results: Biofilms were established on the RBCs and then exposed to 0.7-15 p.p.m. isothiazolones. Young, 1-week-old, biofilms were found to attain treatment efficiency equal to that of mature, 2-month-old, biofilms. Isothiazolone concentrations at 3 p.p.m. and above caused a progressive decline in treatment efficiency and 15 p.p.m. isothiazolones inhibited all microbial activity and resulted in the death of the biofilms. Biooxidation and the biodegradation of isothiazolones within the biofilms continued unhindered at concentrations which caused the total inhibition of planktonic bacteria. Conclusions: There was at least a 10-fold difference in susceptibility of planktonic and biofilm bacteria to isothiazolones. The chemical oxygen demand (COD) test was shown to be a reliable tool for investigating the efficiency of wastewater treatment units when the influent contains isothiazolones, while the biochemical oxygen demand (BOD) test was unreliable due to the inhibition of bio-oxidation by the biocide. Significance and Impact of the Study: The results show that RBCs can be used to treat effluents containing isothiazolones at concentrations up to 15 p.p.m.