This paper discusses restriction fragment analysis and/or sequencing of bacterial 16s rRNA genes, which provide a rapid and convenient method for exploring biofilm community structure regardless of organism culturability. This technique was used to characterize and compare biofilms from several reverse osmosis membranes from a wastewater treatment facility. Special emphasis was placed on detection and identification of Mycobacterium species which were found in previous culture-based studies to be dominant members of the membrane biofilms.