The naphthalenesulfonate-oxidizing bacterium Sphingomonas sp. BN6 was immobilized in calcium alginate. These beads were incubated under aerobic conditions in a medium with the sulfonated azo dye, Mordant Yellow 3 (MY3), and glucose. The immobilized cells converted MY3, but only a marginal turnover of the dye was found under these conditions with freely suspended cells of Sphingomonas sp. BN6. Under anaerobic conditions, suspended cells of Sphingomonas sp. BN6 reductively cleaved the azo bond of MY3 to 6-aminonaphthalene-2-sulfonate (6A2NS) and 5-aminosalicylate. The turnover of MY3 by the immobilized cells under aerobic conditions resulted in the formation of more than equimolar amounts of 5-aminosalicylate, but almost no (6A2NS) was detected. Cells of Sphingomonas sp. BN6 aerobically oxidize 6A2NS to 5-aminosalicylate. It was therefore concluded that the cells in the anaerobic center of the alginate beads reduced MY3 to 6A2NS and 5-aminosalicylate and that 6A2NS was oxidized to 5-aminosalicylate by those cells that were immobilized in the outer aerobic zones of the alginate beads. The presence of oxygen gradients within the alginate beads was verified by using oxygen micro-electrodes. A coimmobilisate of Sphingomonas sp. BN6 with a 5-aminosalicylate degrading bacterium completely degraded MY3. The immobilized cells also converted the sulfonated azo dyes Amaranth and Acid Red 1.