Mycobacterium avium and Mycobacterium intracellulare are environmental opportunistic pathogens whose source for human infection is water and soil. M. avium and M. intracellulare cause pulmonary infections (tuberculosis) in immunocompetent individuals and bacteremia in immunodeficient individuals (e.g. AIDS). One factor likely influencing the lack of success of antibiotic therapy in patients would be their ability to form biofilms. Growth in biofilms might result in antimicrobial resistance because (1) cells are protected by layers of other cells and extracellular material (2) and differences in physiologic state of cells as a consequence of growing on surfaces. The objectives of the work were to (1) establish methods for reproducible growth of mycobacterial biofilms (2) measure the formation of biofilms on surfaces by cells of M. avium and M. intracellulare (3) measure the antibiotic- and chlorine- susceptibility of M. avium and M. intracellulare strain TMC1406T in cell grown in suspension, cells grown in biofilms and suspended and of cells grown in biofilms (4) measure the hydrophobicity of M. avium and M. intracellulare grown in suspension and in biofilms. Methods were developed for growing mycobacteria in biofilms in polystyrene flasks and on glass beads. Although both strains formed biofilms, M. intracellulare strain TMC 1406T more readily formed biofilms than M. avium strain A5 in polystyrene flasks. The majority of M. intracellulare strain TMC 1406T cells grew on the walls of the flasks rather than in suspension like M. avium strain A5. The susceptibility of M7H9 medium-grown cells of M. avium strain A5 and M. intracellulare strain TMC 1406T cells grown in suspension, cells grown in biofilms and suspended and cells grown in biofilms was measured against clarithromycin, ethambutol, kanamycin, rifampicin and streptomycin. Cells grown in biofilms and exposed to antibiotics in biofilms were five-fold resistant to antibiotics than were cells grown in biofilms and exposed in suspension. Cells grown and exposed in suspension were ten-fold more sensitive to antibiotics than were cells grown in biofilms and exposed in suspension. The chlorine susceptibility of cells grown in medium and water was also measured. Cells grown in biofilms were more resistant to chlorine than cells grown in biofilms and suspended. Cells grown in suspension were more sensitive to chlorine than cells grown in biofilms and suspended. The hydrophobicity data (i.e., hexadecane adherence and contact angle measurements) showed that cells grown in biofilms are more hydrophobic than cells grown in biofilms and suspended and cells grown in suspension. It is clear that there are physiological changes between cells grown in suspension, cells grown in biofilms and suspended and cells in biofilms. / Master of Science