Mycobacterium avium is a waterborne opportunistic pathogen commonly detected in drinking water. The resistance of M. avium to common chemical disinfectants at a wide range of temperatures and pH levels allows this microorganism to persist in drinking water distribution systems. The mechanisms responsible for the survival of M. avium in drinking water are not well understood. The persistence of M. avium in distribution systems has been associated to their presence in biofilms or suspended particles inside which these microorganisms appear to gain protection against disinfectant attack. Because chlorine reacts with biofilm or particulate organic matter, the embedded cells are exposed to lower disinfectant concentrations. Previous work (Tong et al., 2002) has shown that a relatively low concentration of monochloramine (< 1 mg/L as Cl2) at 20oC and pH 8 could trigger an adaptive response, which made the bacteria about 2.8 times more resistant than the ones treated with a relatively high concentration of monochloramine (> 5 mg/L as Cl2). Furthermore, preliminary results in this study suggested that once the adaptive response was triggered by exposure to a relatively low concentration of monochloramine, the bacteria were able to present greater resistance to inactivation during subsequent exposure to a relatively high concentration of the disinfectant. This finding can contribute to the explanation of why M. avium can survive in drinking water distribution systems both in planktonic form and in biofilms. Accordingly, the objective of this study was to assess if an adaptive response occurred during the sequential inactivation of M. avium with monochloramine. Materials and methods explained in the paper included: preparation of bacterial suspension; single-step disinfection; sequential disinfection; and, viability assessment. Includes 6 references, figures.