An azo dye-degrading strain, originally named TBX65, was isolated from the mixed liquor of the Mill Creek waste water treatment plant in Cincinnati, Ohio. Strain TBX65 has the unusual ability to aerobically reduce the azo bond of several azo dyes and is able to use some of these dyes as growth substrate. Subsequent investigations have revealed that TBX65 is actually composed of several strains including two azo dye-degrading strains, MC1 and MI2. Strain MI2 is able to use the azo dyes AO7 and AO8 as its sole source of carbon, energy, and nitrogen. In contrast, MC1 can aerobically reduce the azo bond of these dyes but only in the presence of an exogenous source of carbon and nitrogen. Both MC1 and MI2 are Gram negative, rod-shaped bacteria that form yellow colonies. Sequencing and phylogenetic analysis of the 16S rRNA gene of MC1 indicates that it is a strain of Sphingomonas. Based on this phylogenetic analysis, the most closely related strain to MC1 is strain C7, a previously described azo dye-degrading bacterium isolated from biofilms growing in our laboratories. A strain-specific fluorescent antibody has been developed for strains MC1 and MI2, and is being used to determine the survival and azo dye-degrading ability of these strains in biofilms generated in a rotating drum bioreactor.