Microaerobic biodegradation oflow-grade manganese dioxide slurry tailings was demonstrated in enrichment cultures treated with dilute molasses at 30C and pH 58. Supplementary nitrogenous nutrients, yeast extract addition and pH adjustment were not required.Continuously stirred, air-limited reactors were superior to still intermittently agitated cultures, in that, at 5 per cent (w/v) MnOz tailings and 5 per cent (w/v) molasses, the yield of 0.12 g Mn++/g molasses, the rate ofleaching of 106 g Mn++/l/d and a recovery of Mn++ of 58 percent from MnOz were 28 times, 17 times and 19 times greater, respectively. The best compromise in yield (045 g Mn++/g molasses) and rate ofdissolution (073 gMn++/l/d) was at 1 per cent (w/v) molasses in a stirred reactor.Where leaching organisms were separated from sterile ore by a bacteria impermeable membrane leaching was shown to be some 20 per cent due to diffusible metabolites and 80 per cent due to a direct contact between the bacteria and the ore particle's surfaces.